offers: poor interop with other implementations

[AndySchroder]

What doesn't work:

• CLN offers without blinded paths can't be paid by phoenix wallet or LNDK.
• CLN offers with blinded paths routed through LNDK can't be paid by phoenix wallet or CLN.

What works:

• Phoenix wallet offers will receive payments from LNDK and CLN routed through LNDK.
• CLN offers without blinded paths can be paid by CLN routed through LNDK.

Not tested/other issues:

• CLN offers with blinded paths not routed through LNDK paid by phoenix wallet or CLN.
• CLN offers with blinded paths paid by LNDK (doesn't work because of Bug: LNDK doesn't know how to respond to an offer that lists itself as the intro node lndk-org/lndk#123).

Please see lndk-org/lndk#103 (comment) for more details about the testing that I've done across implementations.

[rustyrussell]

Various things contributed to fixing this. Mainly implementing spec updates, but the blinded path fix was the final one.