Add Sequencer-Defined Metering Design Doc and PoC (v0) #360

• Clearly define the problem.
• Introduce the design. Summarizing existing documents with a bit more detail is probably sufficient.
• Include an implementation plan that sketches engineering details for a v0 design.
• Include a benchmarking/evaluation plan that details how the effectiveness of a PoC will be measured.

Existing documents that can be used for inspiration:

• A New Gas Model for the OP Stack
• Fee Rebates