x-pack/filebeat/input/httpjson: refactor/simplify package

[efd6]

The HTTPJSON input has grown to be unwieldy to maintain and debug. To address this we should refactor it to reduce the complication of the code.

Aspects to target:

1. There is significant code non-locality, making it hard to reason about behaviour quickly. This includes similar code in single step processing v multi-step processing (the later derived from the former) living in variously in different files or adjacent.
2. There is significant use of indirection that makes the code more opaque than is truly necessary.
3. It is not clear that the use of concurrency provides a performance benefit and introduces the requirement for locking in a number of places, so look into transforming to sequential processing where possible.

ref: #34929

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)