membership: Restructure pallet into separate files (#4536)

- What does this PR do?
This PR separates `membership` pallet into separate files for `lib`,
`mock` & `tests`.
   
- Why are these changes needed?
Currently `membership` pallet consists of `lib`, `mock` & `tests`
written into a single file.
Refactor it into separate files which makes it inline to other pallets
and improves readability.

- How were these changes implemented and what do they affect?
   The PR will not have any affect.

Signed-off-by: Shreevatsa N <[email protected]>

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Signed-off-by: Shreevatsa N <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>