Add deprecation warning for Google PubSub receivers build in encoder

[alexvanboxel]

Component(s)

receiver/googlecloudpubsub

Describe the issue you're reporting

The Google Cloud PubSub receiver can handle several encodings, but now the receiver can handle external encoders, and we have alternatives for the build in once.

Start deprecation phase 1, and warn the user at startup when they have configured the encoding. Both Google Cloud LogEntry (encoder: #37531) and text encoding have an external encoder alternative and will get the warning log line at start up.

The binary OTLP encoding will still be the default and build in.

[github-actions]

Pinging code owners:

• receiver/googlecloudpubsub: @alexvanboxel

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[crobert-1]

Makes sense to me and was filed by a code owner, removing needs triage