Add new `setup_envs` method for the `Executor` trait

[musaprg]

fixes #2815

This PR introduces a new setup_envs method for the Executor trait. It allows any implementors of the Executor trait to control how environment variables specified in the OCI spec are handled. The default implementation mirrors the behavior of the one implemented in the container_init_process, which involves removing existing environment variables and setting the ones specified in the OCI spec.

TODO

• [x] Make consensus about the interface
• [x] Consider defining a dedicated error instead of ExecutorError
• [x] Add missing test cases

[utam0k]

cc: @Mossaka

[musaprg]

@Mossaka @utam0k @jprendes Hi. Thanks for the reviews! I've updated my PR based on your comments. Would you recheck again?

[utam0k]

@musaprg Congrats! 🎉 Thanks for your contribution.