feat: implement shared client

[oliveigah]

Context: https://github.com/dashbitco/broadway_kafka/issues/132

First attempt to solve the related issue. Let me know what you think! :smile:

Some considerations:

• Currently, for non shared clients, the client_id is harded coded inside the producer module. I've implemented it differently for shared clients, it is returned by the init/1 callback implemented by the client because I need it there in order to return the proper child_spec on the new added callback. Is this difference ok?
• I'm not sure we really need one brod_group_coordinator per producer, but since it is way lighter than the client we can leave it there for now
• When shared_client is false the clients are being started under brod's supervisor but when it is true it is started under broadway supervision tree. Is that ok? I had a tricky bug because of this difference. (left a comment on the code about it)

[oliveigah]

Applied the proposed changes @josevalim!

• Child specs are returned by the brod_client.init callback function
• We always call init on prepare_for_start callback instead of producer initialization

[josevalim]

Tests are failing, can you please take a look? The linting one is quite old, so feel free to bump the Elixir version to make the formatter happier.

[oliveigah]

Tests are failing, can you please take a look?

Sure! I've tested everything locally and everything passed.

Gonna try to reproduce it. Maybe it is flaky somehow.

[oliveigah]

@josevalim, I could not reproduce the integration test problem neither locally or on CI.

But my guess is that using the same topic/group might have impacted in some way, also I increased the waiting time of the integration test, since on the failed run we missed only 1 message (999 expected 1000).

So in summary the changes were:

• Use a new topic and group for the shared client integration test
• Increase the waiting time of integration tests from 1 second to 2 seconds
• Bump elixir lint version to 1.15.6 and erlang version to 26.1.1

[josevalim]

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