Pregenerate SQL for precompiled queries

[roji]

When generating the shaper for a precompiled query, we now check the number of nullable parameters it has, and if that number is low (currently 3), we pregenerate SQLs (or rather, RelationCommands) for it. This PR is stacked on top of #33297 (review only the last commits).

Implementation overview:

• Instead of accepting a RelationalCommandCache (which wraps the 2nd part of the query pipeline), querying enumerables now accept a RelationalCommandResolver, which is a lambda reponsible for producing the relational command.
  • In normal mode, the resolver references the RelationalCommandCache, so things work as before.
  • For precompiled queries with 3 nullable parameters or less, the resolver lambda directly contains code which switches over the nullness of the nullable parameters, and returns captured, fully-built relational commands containing the final SQL string. Example:

var relationalCommandTemplate = ((IRelationalCommandTemplate)(new RelationalCommand(commandBuilderDependencies, "SELECT [b].[Id], [b].[Name]\nFROM [Blogs] AS [b]\nWHERE 0 = 1", new IRelationalParameter[] { })));
var relationalCommandTemplate0 = ((IRelationalCommandTemplate)(new RelationalCommand(commandBuilderDependencies, "SELECT [b].[Id], [b].[Name]\nFROM [Blogs] AS [b]\nWHERE [b].[Id] = @__id_0", new IRelationalParameter[] { new TypeMappedRelationalParameter("__id_0", "@__id_0", relationalTypeMappingSource.FindMapping(typeof(int), "int", false, false, null, false, false, null, null), true, ParameterDirection.Input) })));

SingleQueryingEnumerable.Create(((RelationalQueryContext)(queryContext)), (IReadOnlyDictionary<string, object> parameters) =>
{
    IRelationalCommandTemplate result = default(IRelationalCommandTemplate);
    if (parameters["__id_0"] == null)
    {
        result = relationalCommandTemplate;
    }
    else
    {
        result = relationalCommandTemplate0;
    }

    return result;
}, ...

• This PR also implements awareness of NRT captured variables, so that e.g. a non-nullable string parameter causes only one SQL to get generated (with that SQL being optimized for the non-nullable case), just like for value types:
  • When translating the user's LINQ query from C# to LINQ, the captured variable is represented via an internal fake FieldInfo which contains the NRT information.
  • The funcletizer checks for this information and reports it out, and we store the list of all non-nullable NRT parameters on QueryCompilationContext.
  • When translating ParameterExpression to SqlParameterExpression in RelationalSqlTranslatingExpressionVisitor, we consult this list, and create a non-nullable SqlParameterExpression accordingly.

Closes #29753

[roji]

Since this PR introduces the new CreatePrecompiled() variant below (which accepts the new nonNullableReferenceTypeParameters), this reverts the previous change (this is also good because it leaves precompilation out of non-experimental public APIs).

[roji]

Similarly, this reverts the earlier change and we have separation between the existing public APIs (which are precompilation-unaware) and new, experimental APIs for precompilation.

[roji]

The alternative approach here would be to just put an object instance, and to allow the 2nd part of the query pipeline to throw if it attempts to cast it (e.g. to a collection); we'd simply catch exceptions and assume we can't pregenerate in that case.

[roji]

@dotnet/efteam rebased this on latest main, should be ready for reviewing.