Syslog drain app error messages in app log stream

[corporatemax]

Description

This change should resolve #579 .

I pulled the SyslogConnector.emitLoggregatorErrorLog function into the app_log_emitter.go file and moved its dependent variables accordingly and capsuled it with the AppLogEmitter struct.
Furthermore, I used the AppLogEmitter in the RetryWriter and the WriterFactory to provide error messages to the Application Developer who tries to configure a syslog drain but does not know why there are no logs showing up in the syslog drain target.
I did this also in the FilteredBindingFetcher as there missconfigurations leads to errors there.

Furthermore, to enable this feature, this PR in the cf-deployment repository is necessary, to distribute the TLS config for the forwarder-agent in the binding-cache cloudfoundry/cf-deployment#1301 .

Follow Ups:

• Remove validation logic from filtered binding fetcher or move validation logic to own package to reduce duplication.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Testing performed?

• Unit tests
• Integration tests
• Acceptance tests

Checklist:

• This PR is being made against the main branch, or relevant version branch
• I have made corresponding changes to the documentation
• I have added testing for my changes

If you have any questions, or want to get attention for a PR or issue please reach out on the #logging-and-metrics channel in the cloudfoundry slack