Removing connection_pool field from the CommandProtocol definition

[petyaslavova]

Pull Request check-list

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• Do tests and lints pass with this change?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
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Description of change

With this change, the requirement to initialize connection_pool in every class that directly or indirectly inherits from CommandsProtocol is removed.

The protocol should define only the methods and properties necessary for command execution. Consumers of CommandsProtocol only rely on execute_command, so requiring connection_pool introduces an unnecessary leak of internal state.
In some cases, it's not even applicable—such as with cluster clients, where connection_pool is not a direct property of the client.

Fixes #3574