Add per request functionality to add HTTP request headers

[111andre111]

Feature statement

Adds functionality to configure headers on a per request statement
Syntax example:

<requestname>:
  versions:
    ">= <stackversion>":
      url: "<api url"
      extra-headers:
        <header1>: "<header1value"
        <header2>: "<header2value" 

Checklist

• I have verified that the existing global header functionality in config/diags.yml from Add support for additional headers to DiagnosticService #519 still works
  e.g.

extra-headers:
  elastic-api-version: "2023-10-31"

• I have verified that the local headers in {kibana|elastic|logstash}-rest.yaml overwrite the global header functionality

Fixes #657

[pickypg]

I'm merging this to an intermediate branch (kibana/headers). I'm going to tweak it a little, then bring it into main. Thanks!