Parameterize base fee and max priority fee

[cam-schultz]

Context and scope
The base fee and max priority fee per gas are hardcoded in the relayer source. The hardcoded numbers are conservatively high to increase the chances of the tx being accepted by validators in low traffic scenarios. These same numbers may not be high enough in high traffic scenarios

Discussion and alternatives
We should expose these fee numbers as configuration options for each destination chain. The current hardcoded values can be the default values for these config options.

Open questions
Should we implement a module that dynamically scales these fees according to network activity?