Properly validate whether tranactions are standard

[KonradStaniec]

Add:

• new btcstaking library function to validate whether transactions are standard based on https://github.com/btcsuite/btcd/blob/ee68dc66a835bf2c9333f3d7b33791841f561c84/mempool/policy.go#L285
• use those function to validate unbonding / slashing transactions
• test suite to test new functions

[KonradStaniec]

Does this need to be backported or it can go to the next version?

I wanted to backport it to release patch version v0.12.1, and bump phase-1 services which use this validation: staking-api, and covenant-emulator.

Indexer does not need it as it reads data from BTC ledger, so if miner inserted v0 transaction into the ledger, then indexer must process it