swapserverrpc: arbitrary static swap amount

[hieblmi]

This PR adds an amount field to the ServerStaticAddressLoopInRequest.

    The optional swap amount the client is attempting to swap. If specified the
    server will take out this amount from the total value of provided
    deposit_outpoints and will send the change back to the static address. If
    this results in dust change the server will reject the swap request. If the
    amount is not specified the server will use the total amount of the
    deposit_outpoints as swap amount without providing an additional flag - this
    is due to backwards compatibility.
   

[bhandras]

LGTM 🎉

[starius]

LGTM! 🌴
One comment.

[starius]

What happens, if the amount is too close to the total amount (so the change would be dust)? I think it makes sense if it fails and suggests to remove the flag to use the whole amount.
Could you add the actual expected behavior in this case to the comment, please?

Also, if the amount is equal to the total amount, it makes sense just to succeed, right? And if higher - fail.

[hieblmi]

I expanded the comment to explain.

Note, there is no flag to use the whole amount of provided deposits for backwards compatibility. If deposits are specified without an amount the whole amount is used.