[Argo] helper functions, overflow protection, small fixes

[kdembler]

Scope of changes:

• Introduced few helper functions to limit code repetition in extrinsics and make them more readable
• Added overflow protection in request_outbound_transfer
• Rearranged checks in some extrinsics - we always check bridge status first
• Added check to ensure bridge is paused before calling init_unpause_bridge
• Added tests for:
  • Overflow
  • Trying to request or finalize when the bridge is paused
  • Trying to unpause active bridge
• Removed tests for operator not set - after requiring the bridge to be active first, it's not possible for the bridge to be active and not have an operator at the same time

[ignazio-bovo]

LGTM

[freakstatic]

LGTM!
the helper function were a good idea, looks better now, thanks 👍