Add a field for the network upgrade activation heights to `TestnetParameters`

[arya2]

Motivation

This field is needed for configuring custom testnet activation heights without hard-coded changes or compilation features.

Depends-On: #7968

It addresses part of item 2 in https://github.com/ZcashFoundation/zebra/issues/8280:

First of all TESTNET_ACTIVATION_HEIGHTS[Nu5] constant is modified in network_upgrade.rs#L113 from 1_842_420 to 1_060_755. If there are plans to make this list a config rather than const or somehow use FAKE_TESTNET_ACTIVATION_HEIGHTS

Possible Design

• Add an activation_heights field to the TestnetParameters from #7968
• Add a configuration field in the consensus section for defining custom testnet activation heights or for enabling the ZFuture network upgrade
  • This could also be done separately as #7840

Testing

Check that the parameters have not changed for Network::Mainnet or the default Testnet

[teor2345]

I thought we were going to have a discussion about these tickets first?

[arya2]

I thought we were going to have a discussion about these tickets first?

Yes, I forgot to link the discussion (#7967) in the tracking issue. There was an older comment asking for just this issue.

[teor2345]

I don't think this is required for NU6 specifically?