lnwire: add InboundFee struct in-line within ChannelUpdate

[Roasbeef]

In #6703 the new inbound fee field is added. However, it isn't added directly to the struct, instead the caller must always manually copy the field into the opaque data field, and also must always manually parse it out the other end. We should make this new field uniform with the other TLC fields by adding it directly within the struct. While we're at it, we can also morph it into tlv.OptionalRecord.

[joostjager]

Inbound fees are using a custom tlv id in the high range. I think this would require more flexibility in gen_tlv_types.go?

[Roasbeef]

@joostjager good question, so we can just make entirely new definitions, if we don't want to add 40k extra lines to that file, like so:

type tlvType50000 {}

func (t tlvType50000) TypeVal() {
    50_000
}

func (t tlvType50000) tlv() {}


type TlvType50000 = *tlvType50000

The set up doesn't require that only the code generation be used, just that all new instnaces must be defined in the package.

Not explcitily related, but while we're at it, we can likely simplify from tlv.TlvType5 to tlv.Type5.