Add buyer address and buyable_until

[havan]

This PR:

• Adds buyer address to the mint request, to be used by the supplier bot while minting the BookingToken on chain. The token should only be buyable by this address.
• Adds price info to the mint response to be used by the supplier bot while minting the booking token.
• Adds buyable until into the mint response to be used by the supplier bot to set a expiration timestamp into the token on-chain. It should not be possible to buy the token after this timestamp.
• Updated booking_timestamp field's name, to better reflect what it is, to provider_booking_timestamp. (It's the timestamp of the booking in the supplier's inventory system)

[SamJaarsma]

LGTM

[Noctunus]

LGTM