[Task]: Improve ServerCallContext generation

[guglielmo-san]

Is your feature request related to a problem? Please describe.

No response

Describe the solution you'd like

The current CallContextBuilder does not allow the user to extract their custom user from the Starlette request without redefining the whole CallContextBuilder class.
A simpler callable to extract the user from Starlette Request would be a better approach, keeping a default non-customizable approach to create the ServerCallContext

Describe alternatives you've considered

No response

Additional context

No response

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