feat: cooldown between fee waiver proposals per address

[shakurJJ]

Summary

propose_fee_waiver can be called repeatedly with no cooldown, letting an address spam waiver proposals. This proposes a configurable cooldown between proposals from the same address.

Proposed Behaviour

• Admin sets a cooldown (in ledgers) between successive propose_fee_waiver calls from the same proposer address.
• A proposal submitted before the cooldown elapses since that address's last proposal is rejected with a clear error.
• Default value preserves today's behaviour (no cooldown) unless explicitly configured.

Acceptance Criteria

• Admin can set/get the fee waiver proposal cooldown
• A proposal within the cooldown window is rejected
• A proposal after the cooldown elapses succeeds
• Tests cover both paths

[haryomideolugbojo]

@haryomideolugbojo has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello! I’ve gone through the issue details and acceptance criteria. I’d be glad to take ownership of this task and deliver a clean solution.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @haryomideolugbojo to this issue.