feat(protocol-http): add types for field (#4390)

### Description
Adds `Field`, `FieldPosition`, and `Fields` types to `protocol-http`.

### Testing
`yarn test:all`

packages/protocol-http/src/Field.ts

@@ -0,0 +1,80 @@
+import { FieldPosition } from "./FieldPosition";
+
+export type FieldOptions = { 
+  name: string;
+  kind?: FieldPosition
+  values?: string[];
+};
+
+/**
+ * A name-value pair representing a single field
+ * transmitted in an HTTP Request or Response.
+ * 
+ * The kind will dictate metadata placement within
+ * an HTTP message.
+ * 
+ * All field names are case insensitive and
+ * case-variance must be treated as equivalent.
+ * Names MAY be normalized but SHOULD be preserved
+ * for accuracy during transmission.
+ */
+export class Field {
+  public readonly name: string;
+  public readonly kind: FieldPosition;
+
+  public values: string[];
+
+  constructor({ name, kind = FieldPosition.HEADER, values = [] }: FieldOptions) {
+    this.name = name;
+    this.kind = kind;
+    this.values = values;
+  }
+
+  /**
+   * Appends a value to the field. 
+   * 
+   * @param value The value to append.
+   */
+  public add(value: string): void {
+    this.values.push(value);
+  }
+
+  /**
+   * Overwrite existing field values.
+   * 
+   * @param values The new field values.
+   */
+  public set(values: string[]): void {
+    this.values = values;
+  }
+
+  /**
+   * Remove all matching entries from list.
+   * 
+   * @param value Value to remove.
+   */
+  public remove(value: string): void {
+    this.values = this.values.filter((v) => v !== value);
+  }
+
+  /**
+   * Get comma-delimited string.
+   * 
+   * @returns String representation of {@link Field}.
+   */
+  public toString(): string {
+    // Values with spaces or commas MUST be double-quoted
+    return this.values
+      .map((v) => (v.includes(",") || v.includes(" ") ? `"${v}"` : v))
+      .join(", ");
+  }
+
+  /**
+   * Get string values as a list
+   * 
+   * @returns Values in {@link Field} as a list.
+   */
+  public get(): string[] {
+    return this.values;
+  }
+}

packages/protocol-http/src/FieldPosition.ts

@@ -0,0 +1,4 @@
+export enum FieldPosition {
+  HEADER,
+  TRAILER
+}

packages/protocol-http/src/Fields.ts

@@ -0,0 +1,59 @@
+import { Field } from "./Field";
+import { FieldPosition } from "./FieldPosition";
+
+export type FieldsOptions = { fields?: Field[]; encoding?: string; };
+
+/**
+ * Collection of Field entries mapped by name.
+ */
+export class Fields {
+  private readonly entries: Record<string, Field> = {};
+  private readonly encoding: string;
+
+  constructor({ fields = [], encoding = "utf-8" }: FieldsOptions) {
+    fields.forEach(this.setField.bind(this));
+    this.encoding = encoding;
+  }
+
+  /**
+   * Set entry for a {@link Field} name. The `name`
+   * attribute will be used to key the collection.
+   * 
+   * @param field The {@link Field} to set.
+   */
+  public setField(field: Field): void {
+    this.entries[field.name] = field;
+  }
+
+  /**
+   *  Retrieve {@link Field} entry by name.
+   * 
+   * @param name The name of the {@link Field} entry
+   *  to retrieve
+   * @returns The {@link Field} if it exists.
+   */
+  public getField(name: string): Field | undefined {
+    return this.entries[name];
+  }
+
+  /**
+   * Delete entry from collection.
+   * 
+   * @param name Name of the entry to delete.
+   */ 
+  public removeField(name: string): void {
+    delete this.entries[name];
+  }
+
+  /**
+   * Helper function for retrieving specific types of fields.
+   * Used to grab all headers or all trailers.
+   * 
+   * @param kind {@link FieldPosition} of entries to retrieve.
+   * @returns The {@link Field} entries with the specified
+   *  {@link FieldPosition}.
+   */
+  public getByType(kind: FieldPosition): Field[] {
+    return Object.values(this.entries).filter((field) => field.kind === kind);
+  }
+}