fix(axiom sink): Rebase sink on http sink and remove elasticsearch compatibility

[darach]

Fixes #21292

The axiom sink has been rebased from elasticsearch to http per the issue.
Deprecated headers removed in favor of aligning with the http request configuration
scope, and the compression configuration now defaults to gzip.

The sink now conforms to Axiom's native ingest API:

https://axiom.co/docs/send-data/ingest#ingest-api

The _time field vector event data is no longer rejected. This was a source of
confusion for vector axiom users in the past as elasticsearch semantics were
imposed as the sink was based on the elasticsearch sink and hence field
semantics and constraints.

For existing vector users, the _time field is documented and aligns well with
vectors own timestamp mechanics:

https://axiom.co/docs/reference/field-restrictions#requirements-of-the-timestamp-field

If a @timestamp field is sent to the axiom sink it is now treated as a normal field. This
is a non breaking change.

If a _time field is sent to Axiom it no longer raises a 4XX error, so this is effectively a fix.

As the axiom sink is now HTTP based the full wealth of vectors http configuration apparatus
are now available to users of the axiom sink.

[bits-bot]

All committers have signed the CLA.

[jszwedko]

Thank you @darach ! I'll aim to review this next week.