Add support for Secret Manager

[melbrodrigues]

• Add SecretManagerValueProvider to fetch secrets stored in Secret Manager
• Add --tokenSecretId parameter to include Secret Manager support for Splunk template
• Add --tokenSource parameter to determine Splunk token source

[zhoufek]

Looks like there's a ton of merge conflicts on the attempted import. I think the last commit should be removed. Rather than merging main into this branch, do a rebase and force-push.

It also looks like mvn spotless:apply needs to be run.

[noahmercado]

+1 to needing this

[stale[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[bvolpato]

Thanks for the contributions! I see that contributions are part of the codebase, so I will go ahead and close this PR.