External direct deposits v2

[kilrau]

Is your feature request related to a problem? Please describe.
Currently we are using a two-step process for xud users to deposit funds into a channel

1. send eth/token to user connext client's 0x address
2. from the client, send eth/token to the node by request/rescind deposit rights (open a channel)

Describe the solution you'd like
A way for the user to deposit funds from an external wallet directly to the indra node into a channel in one step with one simple on-chain transaction.

Describe alternatives you've considered
A Loop/Boltz-Style external swap provider. But that seems overkill for something that should be possible natively.

Additional context
We discussed this issue before and the problem is that the connext indra node needs to actively grant deposit rights, otherwise funds can get lost. Spontaneous deposits will likely cause loss of funds.