feat(splunk_hec sink): Add optional force_custom_token parameter to have custom token take priority

[taylorchandleryoung]

Summary

Vector configuration

How did you test this PR?

Change Type

• Bug fix
• New feature
• Dependencies
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

Notes

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[taylorchandleryoung]

Looks like we are going with a more in depth refacotring