WIP: AVRO-3986: [csharp] - Plain JSON encoding for Apache Avro

[clemensv]

[Do not merge.]

This PR accompanies my filing of AVRO-3986 and implements all features defined in the issue. Purpose of the PR is also to illustrate the scope of the work for the other languages. New tests have been added to account for the different modes and the added features.

It's obviously possible to break up the umbrella issue into multiple issues and to break up the PR into separate issues, but since the interoperability goal is only achieved by the sum of the features, I put it all into one. The majority of the new code is related to union handling (features 5 and 6) and RFC3339 date-time handling.

[Xuanwo]

Hi, I believe this PR has been marked as rust in wrong.

[martin-g]

Hi, I believe this PR has been marked as rust in wrong.

The PR diff shows modified files in C#, Java and Rust.
There are 26 commits by several authors (me included) - that makes me think the author needs to rebase to latest main, or create a fresh new branch/PR and cherry-pick only his commits!

[clemensv]

Hi, I believe this PR has been marked as rust in wrong.

The PR diff shows modified files in C#, Java and Rust. There are 26 commits by several authors (me included) - that makes me think the author needs to rebase to latest main, or create a fresh new branch/PR and cherry-pick only his commits!

I bungled a merge during an update and dragged a bunch of stuff in that didn't belong. I just rebased and cleaned that up.

[github-advanced-security]

CodeQL found more than 20 potential problems in the proposed changes. Check the Files changed tab for more details.

[clemensv]

CodeQL found more than 20 potential problems in the proposed changes. Check the Files changed tab for more details.

Interesting. The tool contains about upcasts of the form

(T)(object)((DateTime)logicalValue).ToUniversalTime();

whereby the upcast to object from DateTime is flagged. The cast is required for the cast to (T) to compile and will indeed fail at runtime if there's a mismatch. So that looks like a false positive.