Address CborReader/Writer API review feedback (#37956)

Address CborReader/Writer API review feedback (#37956)

* fix documentation issues

* simplify checks in PeekStateCore

* fix naming issue in CborReader

* apply dirty tricks

* rename CborConformanceLevel to CborConformanceMode

* change default conformance mode to strict

* add cls compliance attributes to offending members

* add CborWriter.Encode(Span<byte>) method

* make CborWriter.WriteStartArray/WriteStartMap methods accept nullable definiteLength parameters

* add WriteTextString(string?) and WriteByteString(byte[]?) methods

* address feedback

* s/None/Undefined/

* replace CborReader.HasData/BytesRead with CborReader.BytesRemaining

* rename Read/WriteCborNegativeIntegerEncoding to Read/WriteCborNegativeIntegerRepresentation

* add a disableConformanceModeChecks parameters to ReadEncodedValue

* make CborReader.PeekState() throw and remove format error enum values.

* move CborReader.SkipValue() implementation to a dedicated source file

* add ReadDefiniteLength(Byte|Text)String() methods

* split string tests into separate source files for each type

* implement CborContentException and use instead of FormatException

* address feedback