[avalanche] Add immature_stake_amount to getavalancheinfo

Summary:
This patch makes the total immature stake amount easily accessible to the user.

Depends on D12086

Test Plan:

ninja
./test/functional/test_runner.py abc_rpc_getavalancheinfo

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12087