add tag parameter to forward output plugin

[fschlager]

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #1165

Does this PR introduced a user-facing change?

Adds the optional `Tag` parameter to fluentbit's output plugin `forward`.

Additional documentation, usage docs, etc.:

[benjaminhuo]

@fschlager Thanks for the PR, don't forget to sign your commit next time:

git commit -s --amend
git push -f