add cloudId and cloudAuth parameters to elastic

[fschlager]

What this PR does / why we need it:

Adds cloudId and cloudAuth parameters to fluentd's elasticsearch output plugin. This enables easier configuration of Elastic Cloud authentication.

Which issue(s) this PR fixes:

Fixes #1168

Does this PR introduced a user-facing change?

Add optional `cloudId` and `cloudAuth` parameters to fluentd's output plugin `elasticsearch`.

Additional documentation, usage docs, etc.:

[fschlager]

I changed the type from string to plugins.Secret, that was an oversight.

[benjaminhuo]

@fschlager Thank you!