feat: option to add mnemonic over api

[arcbtc]

Feels weird not adding auth, but the sidecar isnt designed to be public.
But, still, ideas welcome.

[blackcoffeexbt]

Tested and found the following issue:

server.mjs is not executable so needs a chmod +x server.mjs before the start command as documented works

[arcbtc]

Tested and found the following issue:

server.mjs is not executable so needs a chmod +x server.mjs before the start command as documented works

Done!