[abandoned]route: Add onion blob to sendtoroute

[ErikEk]

This PR adds the ability to parse a custom onion blob to sendtoroute. The blob is limited to 1366 bytes for now.
Partly closes #6206:

• Add a new OnionBlob field to the SendToRoute RPC call. If present, normal path to onion blob compilation can be skipped.
• Add a similar field to SendPaymentV2. In this case path finding should be skipped all together and the onion blob used in place. Note that since we can't decode the error retries as we handle them today aren't possible. Instead we may need some custom retry hooks, or just decide that this feature will be only added to SendToRoute.
• Create provisions in the current APIs to allow for delegated error processing, if we wish to propagate any learnings from the path finding attempt into the internal mission control store.

[bitromortac]

Thank you for working on this, will be a very useful feature! I left a comment in the original issue, happy to know your opinion about it.

[ErikEk]

@bitromortac Thank for letting me know. I'll get back to you.

[yyforyongyu]

Will read @bitromortac 's comments and then come back. Just FYI #6683 will largely change the code here.

[ErikEk]

This is up for grab, right now, if anyone wants to continue the work.

[bitromortac]

!lightninglabs-deploy mute

Disabling the bot until more work is added here.

[lightninglabs-deploy]

@ErikEk, remember to re-request review from reviewers when ready

[yyforyongyu]

!lightninglabs-deploy mute 30d

[SeverinAlexB]

Hi @ErikEk,
The LSPSpec LSPS2 might require sending custom onions. Can you give me an update on the state of this PR?

Thanks
Sev

[ErikEk]

This is up for grab, right now, if anyone wants to continue the work.

Hi,
I did abandon this pr.

[guggero]

@SeverinAlexB feel free to take over the PR if you're interested. Going to close it, the state should be pretty clear from the title and the tags.