feat: merge Broker(context=...) and FastStream(context=...)

[nectarindev]

Description

Fixes #2693

Changes

• New feature (a non-breaking change that adds functionality)
• This change requires a documentation update

Checklist

• My code adheres to the style guidelines of this project (just lint 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 just test-coverage
• I have ensured that static analysis tests are passing by running just static-analysis
• I have included code examples to illustrate the modifications

[CLAassistant]

All committers have signed the CLA.

[nectarindev]

The current implementation merges the broker context into the application context, so a single context will be shared between multiple brokers.
I tried to make the context isolated between brokers, but the mutable design of the library makes this very difficult. If I merge the application context into the broker context, subsequent changes to the application context, such as a modification in app.on_startup, will not be visible in the brokers context.

If the current version is accepted, I would further supplement the documentation.