BLIP update: RFQ wire messages

[dstadulis]

TODO

• blip-29: update RFQ messages Roasbeef/blips#4

Merged

#845 #683 #792 #778

Cover the RFQ implementations in which some slight updates
@ffranr to enumerate those updates in this issue

[ffranr]

The changes I think we should make to RFQ section of the tap BLIP can be found in these reviews that I've left on the tap BLIP PR:

lightning/blips#29 (review)
lightning/blips#29 (review)
lightning/blips#29 (review)
lightning/blips#29 (review)
lightning/blips#29 (review)
lightning/blips#29 (review)

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Summary

I'll create PR(s) for the following RFQ section modifications:

• Expiry field should be a Unix timestamp rather than a "number of seconds until expiry". blip-29: use Unix timestamp for RFQ accepted quote expiry Roasbeef/blips#1
• Add directionality field to the request message type. blip-29: add swap_type field to RFQ request message schema Roasbeef/blips#2
• Or, instead of a directionality field, we could specify both "in" and "out" assets: blip-29: enhance tap_rfq with in_asset_id, out_asset_id Roasbeef/blips#3
• Replace suggested_rate_tick field with BTC amount. (IMO, volume matters in a quote.)
• Add optional error code and error message fields to the reject message type.

Other lower priority modifications:

• Add an expiry Unix timestamp to the suggested BTC amount (currently: suggested_rate_tick) included in the request message.
• Version fields? Or are we just going to bump the message type integer?
• Add asset group key field to the request message type. This will allow us to setup a quote for any asset from the given asset group.
• Other small fixes.

[ffranr]

Corresponding PR: Roasbeef/blips#4

[dstadulis]

Almost ready
can be pulled out of draft soon
@ffranr will apply some small changes

[ffranr]

In this issue, we are implementing the following changes:

• Versioning support.
• Introduction of fixed-point types for the in-asset and out-asset BTC fields.
• Addition of a transfer_type field to the quote request message.
• Expiry will now be represented as a Unix timestamp in seconds.
• Removal of any references to rate ticks; we are now using fixed points.
• The max_amount field in the quote request message is replaced by max_transferable_in_asset.

These changes follow the guidelines outlined in docs/rfq-and-decimal-display.md as detailed here: PR #1059.