fix: race condition when registering grpc types

[abemedia]

What?

Adds a mutex to avoid race condition when registering proto types.

Why?

When registering proto types we first check if they exist. With multiple tests running concurrently the check could result in false for multiple tests which then simultaneously try to register the type, resulting in the following error:

panic: proto: message XXX is already registered

See the following issue:

• gRPC using reflect race on registering types #3961

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

Closes #3961

[CLAassistant]

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