Improve XmlDictionaryWriter UTF8 encoding performance (#73336)

Improve XmlDictionaryWriter UTF8 encoding performance (#73336)

* Speed up text encoding

* Update implementation

* Add tests for binary xml strings

* limit counting code to 256 bit vectors

* reword comment

* rename test

* move bytesmax

* Fix bytesMax after moving variable initialization

* use unicode escape value in test

* fix test typo "*" -> "+"

* Update src/libraries/System.Private.DataContractSerialization/src/System/Xml/XmlStreamNodeWriter.cs

Co-authored-by: Stephen Toub <stoub@microsoft.com>
* Remvoe vectorized code from UnsafeGetUTF8Length

* Fix overfload

* use for loop which seems faster

* remove vector loop

* make sealed encoding to allow devirtualisation

* back some changes

* use uint for UnsafeGetUTF8Chars comparison

* revert more changes

* Fix cutoff based on new measurements

* use BinaryPrimitives.ReverseEndianness as suggested

* Update cutoff from 24 to 32 chars before calling, due to regression for text based DataContractSerializer

* Remove sealed encoding since it only improves XmlConvert

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Co-authored-by: Stephen Toub <stoub@microsoft.com>