Add support for parsing and working with CustomModifiers in field sig…

…natures and make TypeSystemMetadataEmitter more flexible (#70593)

- Update TypeSystemMetadataEmitter to be able to generate a field signature given an appropriate array of EmbeddedSignatureData[]
- Add api to FieldDesc to allow the custom modifier data to be handled using the same scheme as custom modifier data on method signatures
- Adjust TypeSystemMetadataEmitter to be able to generate metadata not just for the Mibc emitter
  - The current implementation has some default behavior around creating metadata that is in the constructor. Break that out into helper functions so that other scenarios don't need to run that code.
  - Add an entrypoint for generating metadata for an arbitrary TypeSystemEntity
  - Add a feature to allow it the ResolutionScope of a non-nested type to be managed specially. This will be needed by #68919

src/coreclr/nativeaot/System.Private.TypeLoader/src/Internal/Runtime/TypeLoader/NativeLayoutFieldDesc.cs

@@ -39,6 +39,8 @@ public override TypeDesc FieldType
             }
         }
 
+        public override EmbeddedSignatureData[] GetEmbeddedSignatureData() => null;
+
         public override bool HasRva
         {
             get

src/coreclr/tools/Common/TypeSystem/Common/FieldDesc.cs

@@ -45,6 +45,9 @@ public abstract TypeDesc FieldType
             get;
         }
 
+        // Get the embedded signature data used to hold custom modifiers and such within a field signature
+        public abstract EmbeddedSignatureData[] GetEmbeddedSignatureData();
+
         public abstract bool IsStatic
         {
             get;

src/coreclr/tools/Common/TypeSystem/Common/FieldForInstantiatedType.cs

@@ -51,6 +51,11 @@ public override TypeDesc FieldType
             }
         }
 
+        public override EmbeddedSignatureData[] GetEmbeddedSignatureData()
+        {
+            return _fieldDef.GetEmbeddedSignatureData();
+        }
+
         public override bool IsStatic
         {
             get

src/coreclr/tools/Common/TypeSystem/Ecma/EcmaField.cs

@@ -113,6 +113,19 @@ public override TypeDesc FieldType
             }
         }
 
+        // This is extremely rarely needed. Don't cache it at all.
+        public override EmbeddedSignatureData[] GetEmbeddedSignatureData()
+        {
+            var metadataReader = MetadataReader;
+            BlobReader signatureReader = metadataReader.GetBlobReader(metadataReader.GetFieldDefinition(_handle).Signature);
+
+            EcmaSignatureParser parser = new EcmaSignatureParser(Module, signatureReader, NotFoundBehavior.Throw);
+            var fieldType = parser.ParseFieldSignature(out var embeddedSig);
+            Debug.Assert(fieldType == FieldType);
+            return embeddedSig;
+        }
+
+
         [MethodImpl(MethodImplOptions.NoInlining)]
         private int InitializeFieldFlags(int mask)
         {

src/coreclr/tools/Common/TypeSystem/Ecma/EcmaSignatureParser.cs

@@ -471,6 +471,25 @@ public TypeDesc ParseFieldSignature()
             return ParseType();
         }
 
+        public TypeDesc ParseFieldSignature(out EmbeddedSignatureData[] embeddedSigData)
+        {
+            try
+            {
+                _indexStack = new Stack<int>();
+                _indexStack.Push(1);
+                _indexStack.Push(0);
+                _embeddedSignatureDataList = new List<EmbeddedSignatureData>();
+                TypeDesc parsedType = ParseFieldSignature();
+                embeddedSigData = _embeddedSignatureDataList.Count == 0 ? null : _embeddedSignatureDataList.ToArray();
+                return parsedType;
+            }
+            finally
+            {
+                _indexStack = null;
+                _embeddedSignatureDataList = null;
+            }
+        }
+
         public LocalVariableDefinition[] ParseLocalsSignature()
         {
             if (_reader.ReadSignatureHeader().Kind != SignatureKind.LocalVariables)

src/coreclr/tools/Common/TypeSystem/IL/Stubs/PInvokeLazyFixupField.cs

@@ -56,6 +56,8 @@ public override TypeDesc FieldType
             }
         }
 
+        public override EmbeddedSignatureData[] GetEmbeddedSignatureData() => null;
+
         public override bool HasRva
         {
             get

src/coreclr/tools/Common/TypeSystem/Interop/IL/InlineArrayType.cs

@@ -441,6 +441,7 @@ public override TypeDesc FieldType
                     return _owningType.ElementType;
                 }
             }
+            public override EmbeddedSignatureData[] GetEmbeddedSignatureData() => null;
 
             public override bool HasRva
             {

src/coreclr/tools/Common/TypeSystem/Interop/IL/NativeStructType.cs

@@ -348,6 +348,8 @@ public override TypeDesc FieldType
                 }
             }
 
+            public override EmbeddedSignatureData[] GetEmbeddedSignatureData() => null;
+
             public override bool HasRva
             {
                 get