Add proof courier address field to Tap address

[ffranr]

A Tap address should include a mandetory proof courier address.

The receiving node during the asset send procedure could use this new field to contact a proof courier service and retrieve proof(s) associated with then send event.

In other words: In order to receive and asset, the receiver generates a tap address. That tap address will contain a proof courier address. The sender submits the transfer proof to that proof courier. Which the receiver can then retrieve.

This new field should support multiple proof courier protocols:

universerpc://the.host.name:port
hashmail://mailbox.server:port

This is a prerequisite to #344

[ffranr]

PR for adding TLV field: #450

[guggero]

Fixed by #450.