[avalanche] Add the staking rewards to the coinbase outputs

Summary:
This actually adds staking rewards to the miner. This is hidden behind a flag and disabled by default so that it's opt-in and doesn't change the behavior if unset.

Depends on D14437.

Test Plan:

ninja all check-all

Check the getblocktemplate doc:

bitcoin-cli help getblocktemplate

Reviewers: #bitcoin_abc, PiRK, bytesofman

Reviewed By: #bitcoin_abc, PiRK, bytesofman

Subscribers: bytesofman, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14438