[avalanche] Add an easy to use shortcut flag to turn on avalanche

Summary: This is a convenience flag, which still let the user override the flags he wants. This depends on D11358 for the min stake amount to work properly. There is no change in behavior when the flag is not set.

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, tyler-smith

Reviewed By: #bitcoin_abc, tyler-smith

Subscribers: tyler-smith

Differential Revision: https://reviews.bitcoinabc.org/D11359