Bracket syntax for accessing dotted field names in ingest processors

[felixbarny]

Allows accessing fields that have dots in their name via a bracket syntax.
For example: attributes['foo.bar'] or attributes["foo.bar"]

Open questions:

• Should we also allow or require omitting quotes like attributes[foo.bar]?
• Do we need an escape syntax?
• Should we add a way to access array elements, such as resource.attributes['host.ip'][0]? Probably not now, we can add that later if needed.
• Does anyone rely on being able to access fields with [ or ] in them? What about [', [", "], or ']?
• Does this need an opt-in? That will depend on the answer of the question above.
• Do we want to require users to explicitly define where the dotted fields are or should we try different combinations with attributes.foo.bar? I'd say maybe, even probably. But that's a separate thing which should have a separate syntax or way of opting in.

[elasticsearchmachine]

Hi @felixbarny, I've created a changelog YAML for you.