Move into shared for SqlCachedBuffer.cs

[lcheunglci]

Relating to issue #1261 . I moved the SqlCacheBuffer.cs from netcore to shared src and updated the reference in the netfx csproj. I did notice that in the ToXmlReader method was implemented differently between the two. I looked at the helper method SqlTypeWorkarounds.SqlXmlCreateSqlXmlReader(...) and it seems to be doing the same functionality without using reflection.

[lcheunglci]

I decided to remove the ToXmlReader method because there's no references to it anywhere in the code and tests. After further comparison between the reflection implementation in netfx and the helper method in netcore, they both seems different but since it's an internal method and it's not being used, I feel like it's safe to remove.

[Wraith2]

Seems reasonable. We'll see if it affects any of the tests (which sometimes use private reflection) and if nothing is broken then it'll be a good cleanup.

[Wraith2]

Unless there are merge conflicts I wouldn't suggest keeping all your PR's up to date with main. It will take you a lot of time and it can be months before the team are ready to review.

[DavoudEshtehari]

Modifiers could be reordered on ToString(): public override string ToString()

[DavoudEshtehari]

NIT: It would be simpler using expression body syntax:
public bool IsNull => _cachedBytes is null;

[DavoudEshtehari]

Extra lines; could be removed.

[DavoudEshtehari]

NIT: This way it would be neater.
internal List<byte[]> CachedBytes => _cachedBytes;