watch_staking endpoint accepts unsent transaction

[KonradStaniec]

Currently watch_staking endpoint (https://github.com/babylonchain/btc-staker/blob/main/stakerservice/service.go#L434) will hapilly accepts staking transactions which weren't sent to btc by the caller. This can lead to possiblity of dosing staker program.

There are few possiblities to fix it:

• we can have timeout for waiting for confirmations i.e if we not receive 1st confirmation in 30min, we cancel whole thing
• we can have staker, first to check its node mempool for the staking tx, and if its not there in like 5min from receving staking request then, cancel whole thing
  -we can require staker to send valid staking tx (i.e signed and ready to be send to btc) and just send it by itself. Then if tx is already in staker btc node mempool we just declare everything is correct, and if not it will just speed up staking tx propagation through the network as it will propagate from staker node and from whatever the wallet sent it.

Drawbacks of every of this solution is that we start relaying on btc node attached to staler program, which can lead to rejecting some staking requests if btc node is not synced or lost data.