[feature]: Allow transactions with a lower fee rate than 1 sat/vbyte

[ziggie1984]

Currently, lnd enforces a minimum transaction fee floor of 253 sat/kw (equivalent to ~1 sat/vbyte), which prevents users from setting lower fee rates even when network conditions might allow for cheaper transactions.

Bitcoin Core is working on allowing propagation of sub-1 sat/vbyte transactions in PR #33106: bitcoin/bitcoin#33106

LND should also allow transactions which are lower than 1 sat/vbyte.

[ellemouton]

@ziggie1984 - and just checking, we would need to gate this on the version of the bitcoind backend yeah? cause i assume with an older bitcoin node, such a tx would be rejected?