[processor/transform] Add support for flat configuration style

[edmocosta]

Description

This PR is part of #29017, and a split from #36888. It changes the transformprocessor, adding support for flat configuration styles.

Change log:

• It now supports an additional configuration style, where statements are expressed as a list of strings, being the path's context required, and the context inferred from them thanks to the context inferrer & ottl.ParserCollection. For example:

  log_statements:
   - set(log.body, "bear") where log.attributes["http.path"] == "/animal"
   - set(resource.attributes["name"], "bear")

• It does support mixed configuration styles.
• The context's cache values are only shared among flat statements
• Structured configuration cache values are still isolated, which means that a cache written using a structured configuration style will only be available for that configuration group's statements, and won't be shared with flat statements and/or other structured configuration groups, for example:

  log_statements:
  - set(resource.cache["flat"], "value")
  
  -  statements:
     - set(resource.cache["name"], "bear")
     - set(resource.attributes["name"], resource.cache["name"]) # OK
     - set(resource.attributes["name"], resource.cache["flat"]) # Fail(not set by this group of statements)
  
  - set(resource.attributes["name"], resource.cache["name"]) # Fail(not set by a flat statement)
  - set(resource.attributes["flat"], resource.cache["flat"]) # OK
  
  -  statements:
     - set(resource.attributes["name"], resource.cache["name"]) # Fail(set by another group)

Link to tracking issue

#29017

Testing

Unit tests

[TylerHelmuth]

@evan-bradley please review

[evan-bradley]

Sorry for the long delay. The way we're handling the config makes sense to me.