Make the halving interval configurable for custom networks and regtests

[oxarbitrage]

There had been a request for making the halving heights configurable in Regtest mode:

https://discordapp.com/channels/809218587167293450/1285973514964439121/1285975294997434369

For the first halving, we have a TODO here to do this: https://github.com/ZcashFoundation/zebra/blob/v1.9.0/zebra-chain/src/parameters/network/subsidy.rs#L394

Also, this feature will be useful if we plan to merge https://github.com/zcash/zcash/blob/v5.10.0/qa/rpc-tests/nuparams.py into the zebra rpc test suite.

[oxarbitrage]

Posted by @arya2 in discord:

I've just realized that the halving interval is much shorter in zcashd's regtest, (see https://github.com/zcash/zcash/blob/master/src/consensus/params.h#L251-L252), we should make the halving interval configurable on custom Testnets too and set it to 144 on Regtest, in Zebra those constants are currently here: https://github.com/ZcashFoundation/zebra/blob/restore-internal-miner/zebra-chain/src/parameters/network/subsidy.rs#L38-L45)

So we could split this ticket in 2 tasks:

• Use a halving interval of 144 in regtest mode.
• Make halving interval configurable in custom testnets.

Making the first will unblock porting https://github.com/zcash/zcash/blob/v5.10.0/qa/rpc-tests/nuparams.py into zebra.