Endpoint to trigger contract recovery

NEW FEATURE

Description of Feature

This MR adds a new API endpoint which allows the user to trigger a rescan of the blockchain to find recoverable file contracts.

Description of Work Flow

The user calls the POST endpoint /renter/initrecovery which starts a recovery thread in the background if the wallet is unlocked. (Otherwise the contractor can't fetch the seed)

Testing Added

TestRenterContractInitRecoveryScan

Modules and Packages Impacted

renter, renter/contractor, siatest/renter