Add ability to create channel with external funding

[benthecarman]

The other major lightning implementations have the ability to open a channel from an external transaction. It would be nice to have this ability to be done in eclair.

[t-bast]

Duplicate of #1565, this overall feature should encompass giving you more freedom about what utxos to use for channel open.
It hasn't made much progress yet for two reasons:

• we need to introduce PSBT utilities inside the codebase first (in progress with Use psbt to fund and sign transactions #1876)
• we need to first implement dual-funding and see how it will impact the funding flow