Add metadata fields to elastic adapter

[JSCU-CNI]

This PR adds metadata fields to the elastic adapter and repairs elastic+[PROTOCOL]:// behaviour. It also enables users to authenticate to Elasticsearch with an API key.

You can now write arbitrary metadata to the document._source._record_metadata dict using the following syntax:

rdump -w "elastic+https://localhost:9200?_meta_foo=bar"

This will result in the following _record_metadata dict:

{
    ...
    "foo": "bar"
}

Of course we can change _meta_ to something else if that seems more appropriate for the flow.record project.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 76.92308% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 83.67%. Comparing base (e0586ef) to head (cd99f9b).

Files	Patch %	Lines
flow/record/adapter/elastic.py	76.92%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #121      +/-   ##
==========================================
+ Coverage   81.94%   83.67%   +1.72%     
==========================================
  Files          34       34              
  Lines        3385     3394       +9     
==========================================
+ Hits         2774     2840      +66     
+ Misses        611      554      -57     

Flag	Coverage Δ
unittests	83.67% <76.92%> (+1.72%)	⬆️

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[yunzheng]

Just noticed some small inconsistencies for the http:// prefixing. See code suggestions.

After this it should be good to merge!