Add tx_timestamp to resultset in api/outputs

[ghost]

We would like to have tx_timestamp added as an output from the api/outputs JSON API which Bisq intends to use for XMR tx proof integration.

This would enable the caller to validate that the trade date is not older than expected.

Related discussion

// TODO check if date of tx is after tradeDate (allow some tolerance to avoid clock sync issues).
//  Otherwise a scammer could send an old txKey from a prev trade with same amount before seller has
//  updated to new feature, thus the check for duplication would not detect the scam.
//  >>>>> the API does not provide date of tx <<<<<<

bisq-network/bisq#4421 (comment)

Testing performed

• Check tx_timestamp when transaction is in mempool (was 1598054064)
• Check tx_timestamp when transaction is first included in a block (was 1598054187)
• Check tx_timestamp when transaction has multiple confirms (was 1598054187)

/api/outputs?txhash=b87c96c1d7f121a3f27d5a59211de8f5f21b2a110ffa3586f91b6e21232e30e9&address=4ATyxmFGU7h3EWu5kYR6gy6iCNFCftbsjATfbuBBjsRHJM4KTwEyeiyVNNUmsfpK1kdRxs8QoPLsZanGqe1Mby43LeyWNMF&viewkey=8152a4a2fa17c66acfb91e35f1699f7252f1a66cf963fd94da3776ee09a9aa0d&txprove=1

[moneroexamples]

Thank you.