Adding nullable type functionality for HttpRequest Context Type (#32203)

* Added "?" to make type string nullable for ContentType.  Updated PublicAPI.Unshipped.txt to reflect change to nullable type.

* Added *REMOVED* counter part to addition as requested.

* Added "?" to update DefaultHTTPRequest ContentType property to allow nullable.

* Added null value marker for EscapedValueOrEmptyMarker function used by ToString function.

* Updated contentType paramater to include null values in TryGetEncodingForMediaType function.

* Implemented TryParse on content type.  If false, returns false for TryEncodingForMediaType.  If true, continues through Encoding process.