[testnet] BridgeHubRococo nits (#1972)

This PR does not introduce any functional changes to the existing code,
it merely addresses several minor refactors:
- Moving bridging pallets to separate files.
- Improving the readability and naming of weight files for bridging
pallets and bridging pallet instances.

The reason for this refactor is to facilitate easier plugin integration
for the upcoming bridge between Rococo and Westend.

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Co-authored-by: command-bot <>