Max htlc option for fee adjuster

• We can reduce routing failures by liming the maximum size of outbound HTLCs through a channel
• This change adjusts the max HTLC according to the current liquidity available on a channel.
• I have added this to the feeadjuster plugin to reuse its trigger mechanism and channel balance logic
• The default is for the feature to be turned off so it will not cause unexpected behaviour changes for plugin users who update
• I have also added a basic readme