(fix, csharp): map<string, unknown> values are nullable

[amckinney]

This adds special handling for the map<string, unknown> type so that we support null values in arbitrary metadata which is frequently typed as Dictionary<string, object?>. This is particularly useful for patch semantics, which require removing some piece of information entirely, i.e.

{
  "name": "example",
  "tags": null
}

A couple other lightweight changes are planned for the upcoming minor release, so this includes the release notes in an Unreleased section.