Ability to sanitize headers in the `:timber_plug` integration

[binarylogic]

As part of splitting out the libraries we need to support the :header_keys_to_sanitize option in the new :timber_plug library. Previously this was defined as a root level timber configuration, but this should be moved to the :timber_plug library instead:

config :timber_plug, header_keys_to_sanitize: ["authorization"]

Note, this should be case insensitive, which I think plug headers are anyway. It's probably best to just move this method into that library:

https://github.com/timberio/timber-elixir/blob/948be10c6416a77b1c27f93a598ae7113a27f80b/lib/timber/utils/http_events.ex#L78

[valpackett]

Would be nice to be able to completely turn off the inclusion of headers there too.

...also why are they headers_json? what's the purpose of nesting JSON-in-JSON or JSON-in-msgpack?