feat: allow collateral top-ups

[emizzle]

During a started request, SPs may be slashed if they miss enough proofs. Currently, there is no way for them to top up their collateral so they are in a "full collateral" state. This weakens the security of the protocol.

Collateral for started requests should be allowed to be topped up, with the following constraints:

1. The contract must be started
2. The collateral cannot exceed the total collateral required for the request
3. A valid storage proof must be submitted in the transaction to top up the collateral