Implement prover escrow

[just-mitch]

#8572 created a skeleton escrow contract for holding prover commitment bonds in Test token (TST): its implementation is filled with no-ops, and no escrowing or authorization is required.

This issue is concerned with ensuring that:

Tasks

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1. Provers must deposit TST prior in order for their Quotes to be redeemable
2. Quotes must have valid signatures from provers
3. Bonds can be staked/unstaked by the rollup
4. Provers can withdraw TST with a delay of 3 epochs