Add static labels support for the AWS Firehose Logs

[andriikushch]

PR Description

Adds support for setting some static labels at the AWS configuration level.

Static labels can be set with the X-Amz-Firehose-Common-Attributes header.

Label names should be prefixed with lbl_. Label names and label values should be compatible with a Prometheus data model specification.

Example of the valid X-Amz-Firehose-Common-Attributes value with two custom labels:

{
  "commonAttributes": {
    "lbl_label1": "value1",
    "lbl_label2": "value2"
  }
}

Which issue(s) this PR fixes

N.A.

Notes to the Reviewer

N.A.

PR Checklist

• [x] CHANGELOG.md updated
• [x] Documentation added
• [x] Tests updated
• [ ] Config converters updated

[andriikushch]

Thank you, @clayton-cornell for the suggestions 👍

[clayton-cornell]

Doc content looks OK to me. It's over to @grafana/grafana-alloy-maintainers for code review now :-)