Add multisig discovery field to account data

[andywer]

Need to set some account data when making an account multi-sig.

A multisig account adds a data entry with the key config.multisig.domain and the value of a hostname used to resolve the stellar.toml file of the multisig service.

Example: Key: config.multisig.domain Value: stellarguard.me

See SEP-19.

[andywer]

Moving back out of backlog, since the SEP proposal is still being discussed right now.

Edit (May 2020): It's SEP-19 nowadays. Updated the description to reflect that.