Add support for Key-Value pairs

[ellttBen]

Hi there, As discussed in #329, this PR implements support for the log::kv structured logging API. These are some user-facing choices I have made:

• Everything is implemented behind a log_kv feature
• Support is added in the two encoders json and pattern:
  • The json message has an attributes field which is a serialized map of the record's structured logs
  • The pattern encoder has a new K or key_value formatter which functions exactly like the mdc formatter.

If this sounds okay, I can also edit the documentation.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 91.17647% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 64.09%. Comparing base (8ab1b34) to head (24257cf).

Files	Patch %	Lines
src/encode/pattern/mod.rs	88.46%	3 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #362 +/- ## ========================================== + Coverage 63.39% 64.09% +0.70% ========================================== Files 24 24 Lines 1557 1593 +36 ========================================== + Hits 987 1021 +34 - Misses 570 572 +2 ```

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[ellttBen]

Hi, sorry for the delay, I'm still keen to get this merged. I have rebased this PR and squashed the commits. If more documentation is needed I can get that in ASAP as well.

[bconn98]

I'll take a look at this soon. I'll do my best to get a hold of the project owner and work through our recent batches of PRs

[sdroege]

What's the status of this PR?

[bconn98]

@sdroege I am targeting next week to start working with the owner of the repo. Appreciate everyone's hard work on the recent batch of MRs and can't wait to get things moving again!

[sdroege]

That sounds great, thanks!

[ellttBen]

Thanks for the review @estk, I have followed your suggestion and investigated directly playing around with Serde traits. I implemented the suggested review changes, and also changed the approach for json encoding to directly (via a new wrapper type) serializing from the &log::kv::Source dynamic reference. The error handling is a bit finicky with transformations between log::kv::Error and the serializer errors (log::kv::Error::boxed requires Send+Sync, and the serde traits make no such guarantees about allowable errors), but I think overall it makes more sense.

[estk]

blocked on #367