"extra_amount" option in name_new/firstupdate/update

[JeremyRand]

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

As per consensus rules, name outputs can have any amount as long as it is at least 0.01 NMC, but this is not exposed via the name_new, name_firstupdate, and name_update RPC methods. This is unfortunate, as this is an important use case for saving fees and atomic name trades.

Describe the solution you'd like

Add an extra_amount field to the options parameter, which defaults to 0. The amount used for the name output will be equal to the minimum name output amount (currently 0.01 NMC) plus the value of extra_amount.

Describe alternatives you've considered

Considered the raw transaction API, but this is not reasonable UX.

Additional context

N/A.