RFC for ZIP-213: Validate Shielded Coinbase Outputs

[mpguerra]

Is your feature request related to a problem? Please describe.

ZIP-213 introduces shielded coinbase.

Describe the solution you'd like
In order to implement this we should first write up the design for how this work in an RFC.

Additional context

PR #1190 started implementing this work and should be used to inform the design and write up

[mpguerra]

Moving this out of Sprint 13 as we'll deal with coinbase rules at a later date, post NU5 activation

[teor2345]

Moving this out of Sprint 13 as we'll deal with coinbase rules at a later date, post NU5 activation

We're only skipping coinbase input validation: Founders Reward, Funding Streams, and Miner Rewards.
We have to validate coinbase outputs, because outputs get spent along with other Zcash coins.

But looking at the list of consensus rules in #608, we don't need a design here, just some good function and module docs.