Add `maxClosingFeerate_opt` to `DATA_CLOSING`

[t-bast]

We introduced a max-closing-feerate option in #3097 to ensure that RBF attempts for "main" closing transactions (transactions that are not at risk of being double-spent by our channel peer) don't end up being too much on-chain fees when we're not in a hurry to get our funds back.

However, in some cases, we want to close specific channels with a different upper bound. We could achieve that by adding a maxClosingFeerate_opt field to DATA_CLOSING: when not provided, we would use the default max-closing-feerate from eclair.conf. We would add this argument to the forceclose RPC, which would let node operators specify it on a per-channel basis. This would also let node operators change that feerate by calling the forceclose RPC again with a different value.

[t-bast]

#3118 adds a field in DATA_CLOSING that we can then use to implement this feature.