Add option to provide SIP credentials

Currently the only supported way to get SIP credentials for JMP after initial signup process is documented in https://jmp.chat/#calling as:

If you don't have your SIP account information, you can reset your SIP account and receive the new password and other details using an XMPP ad-hoc command. For example, you can do this on a computer using your JID in Movim and then clicking on cheogram.com in your contacts. Under Actions, you can then choose "Reset SIP Account (if forgot password)". The next page will show you your new SIP password as well as the SIP username and domain you can use to login.

Ideally the bot should allow retrieving the current credentials. If that is not possible, then providing a bot-based method of resetting the credentials (with confirmation as it's potentially destructive) may be a reasonable alternative.