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.