feat(fluentd-plugin-datadog-loki): support custom http headers

[AndrewChubatiuk]

What this PR does / why we need it:

PR adds ability to set custom headers, this allows to send extra information, like custom auth methods or any other additional headers, which may contain additional metadata

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[CLAassistant]

All committers have signed the CLA.