refactor: allow to edit the file listings before copy

[AlexisBRENON]

Split the execute method to move the "file listing" retrieval to a dedicated method.
This allow to apply arbitrary modification of file listing more easily.

My use case is to sample the file listing (to avoid to copy too much file on a run, just copy 1/1000).

With the previous implementation, I had to override the whole execute method, to be able to manipulate the s3_objects list before passing it to the transfer methods.

class MyOperator(S3ToGcsOperator):

    def execute(...):
        # [...]
        s3_objects = S3ListOperator.execute(self, context)  # Update here to call the right method
        s3_objects = sample(s3_objects)  # Add my sampling method

        # [...] Copy the rest of the method, keeping it in sync with upstream

With the new implementation, I can only override the _get_files one, mostly calling the super method and just modify the list before return.

class MyOperator(S3ToGcsOperator):

    def _get_files(...):
        s3_objects = super()._get_files(...)
        return sample(s3_objects)  # Add my sampling method

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[jason810496]

The static check needs to be fixed.
You can quickly fix it by running:
pre-commit run ruff --all && pre-commit run ruff-format --all.