[feature]: embed flag for assets send api

[lukegao209]

Is your feature request related to a problem? Please describe.

Add an embed flag to allow users to include an additional vout in transactions, which can be used to tag business operations and expand more possibilities.

Describe the solution you'd like

tapcli assets send --addr xxxxx --embed 'hello world'

[Roasbeef]

I take it this would be an OP_RETURN or something else?

[lukegao209]

I take it this would be an OP_RETURN or something else?

yeah, like this in bitcoin-js sdk const embed = bitcoin.payments.embed({data: [data]}) key_spend_psbt.addOutput({ script: embed.output, value: 0 // （OP_RETURN） });