feat: fees on listpeers rpc output

[m-schmoock]

As mentioned in the last developer meeting and additional to #4221 we want to have the local half-channel fees (the ones that are applied when a payment is routed though our node) as part of the listpeers output. Motivation is that this data is non-gossip and currently only available via gossip listchannels.

Example

{
    ...
    "fee_base_msat" : "100msat",
    "fee_proportinal_millionths" : 1000,
    ...
}

This PR adds feature, test and doc.

[ZmnSCPxj]

ACK 8dbe6f1

Minor quibble about docs but not enough to block this.

[rustyrussell]

OK, so minor feedback:

1. We prefer to use spec names for fields (unless we've already misuse elsewhere). That makes it fee_base_msat and fee_proportional_millionths.
2. msat field names should end in _msat so that the Python JSON decoder can automagically turn them into Millisatoshi amounts.

[m-schmoock]

@rustyrussel
Thx for the feedback. I changed the names.

I figured that in the current listchannels output base_fee_millisatoshi is currently not having a _msat pendant. Should we add this in another PR? If we want to eventually get rid of non-msat fields we need to add this.

[m-schmoock]

@rustyrussell

1. We prefer to use spec names for fields (unless we've already misuse elsewhere). That makes it fee_base_msat and fee_proportional_millionths.

Note: Just checked, listchannels gives me fee_per_millionth ...

[cdecker]

ACK 54af4c9