feat(otelcol/exporter/splunkhec) Add Splunk HTTP Event Collector (HEC) Exporter

[PatMis16]

PR Description

This PR initiates the inclusion of the OTel Splunk HEC exporter component to Grafana Alloy as a community component.

Which issue(s) this PR fixes

Fixes #1226

Notes to the Reviewer

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated
• Config converters updated

[wildum]

Hey, thanks for your interest in the project :) Could you first create a proposal for the component please? (see https://github.com/grafana/alloy/blob/main/docs/developer/adding-community-components.md).
It's a good candidate for community components but because it will be part of the Alloy binary we still need consensus from the team before starting the implementation

[PatMis16]

Created the proposal: #1646

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