Add fluent combinators for adding context to structured loggers

[nigredo-tori]

Closes #424

• addContext(Map[String, String]) members are trivial wrappers over existing withContext functions. I couldn't call this withContext because for some reason that disabled static forwarders for StructuredLogger.withContext etc., which broke binary compatibility. 🤷 Thankfully, I like the new name better than the old one, because it makes it clear that the context is appended rather than replaced.
• addContext((String, Shown)*) is sugar over it using the Show.Shows magnet. Consider:

  logger.addContext("foo" -> 123)
  // versus
  logger.addContext(Map("foo" -> 123.show))

We can add similar sugar for error, info etc. - but I'm not sure those would be useful enough to justify the required explosion of the number of methods.

[lorandszakacs]

I do like the idea, and it has my OK. But before that:

1. the feature should branch off of series/1.x branch, and we forward port it to main series 2.x
2. the method should exist on StructuredLogger[F[_]], and then override it in SelfAwareStructuredLogger[F[_]] as well. The problem is analogous to the withModifiedString method, fixed in Add missing withModifiedString overrides in Logger[F] subclasses #430

[nigredo-tori]

1. the feature should branch off of series/1.x branch, and we forward port it to main series 2.x

I've rebased this PR to 1.x, with no conflicts.

2. the method should exist on StructuredLogger[F[_]], and then override it in SelfAwareStructuredLogger[F[_]] as well. The problem is analogous to the withModifiedString method, fixed in Add missing withModifiedString overrides in Logger[F] subclasses #430

Could you please elaborate? As far as I see, this is already OK - both methods are introduced in StructuredLogger[F[_]] (returning StructuredLogger[F]), and both are explicitly overriden in SelfAwareStructuredLogger[F[_]] (returning SelfAwareStructuredLogger[F]).

[lorandszakacs]

Could you please elaborate? As far as I see, this is already OK - both methods are introduced in StructuredLogger[F[_]] (returning StructuredLogger[F]), and both are explicitly overriden in SelfAwareStructuredLogger[F[_]] (returning SelfAwareStructuredLogger[F]).

You're right, sorry. Must have overlooked 😅 all good 👍