Separated thread for confluent kafka consumer client.

[DABND19]

Description

Fixes #1904

All consumer operations are guaranteed to run in a separate thread. This allows us to no longer use asyncio.Lock, and also does not block threads from the shared pool.

Type of change

Please delete options that are not relevant.

• Documentation (typos, code examples, or any documentation updates)
• Bug fix (a non-breaking change that resolves an issue)
• New feature (a non-breaking change that adds functionality)
• Breaking change (a fix or feature that would disrupt existing functionality)
• This change requires a documentation update

Checklist

• My code adheres to the style guidelines of this project (scripts/lint.sh shows no errors)
• I have conducted a self-review of my own code
• I have made the necessary changes to the documentation
• My changes do not generate any new warnings
• I have added tests to validate the effectiveness of my fix or the functionality of my new feature
• Both new and existing unit tests pass successfully on my local environment by running scripts/test-cov.sh
• I have ensured that static analysis tests are passing by running scripts/static-analysis.sh
• I have included code examples to illustrate the modifications

[DABND19]

1. https://github.com/airtai/faststream/blob/0.6.0/faststream/kafka/subscriber/usecase.py#L144
2. https://github.com/airtai/faststream/blob/0.6.0/faststream/rabbit/subscriber/usecase.py#L137