CIP 18: Add tx priority to requests

[cmwaters]

This PR makes a few minor modifications to CIP 18: Standardised Gas and Pricing Estimation Interface

• Remove the reference implementation section. Client and server implementations in the light and consensus node respectively will have their own ADRs that eventually can be linked in this CIP
• Add an optional TxPriority field to indicate the urgency for a transaction to be finalized in a block. It has three levels: low, medium, and high.
• Modify the APIs to have two methods: requesting just the gas price and requesting the gas price and the gas. The latter will be used for non-PFB transactions.

[rach-id]

implementation related questions that can be moved to an ADR if desired

[jcstein]

LGTM