Tracking: Add parameters to `Network::Testnet` variant

[arya2]

Motivation

Zebra currently hard-codes some consensus parameters that may need to be changed for testing.

Scope

To be discussed in https://github.com/ZcashFoundation/zebra/discussions/7967.

Preparatory Refactors:

• [x] #8325
• [x] #8326
• [x] #7968

After these changes are merged, we should decide which parameters we want to add and create more issues like #7970 for each of them.

Adding Parameters:

• [x] #7970
  • [x] #7840
• [x] #8361
• [x] #8362
• [x] #8327
• [x] #8412
• [ ] Add a default_port field to NetworkParameters (needed for custom Testnets but not Regtest)
• [ ] #8364
• [x] #8365
• [x] #8366
• [ ] #8367

Possible Design

• Turn the Testnet variant into:

Testnet(Arc<NetworkParameters { 
   cache_name,
   genesis_hash,
   network_id,
   default_port,
   activation_heights,
   ..
>)

• Consider adding fields for:
  • slow_start_interval (nSubsidySlowStartInterval)
  • target_difficulty_limit (powLimit)
  • key_prefixes
  • debug_mandatory_checkpoint_height: Option<Height>
  • checkpoints: Path
  • pow_avg_window (update logic to use min(num_blocks_in_cache, 17))
• Add a new_testnet() method or DEFAULT_TESTNET constant with the existing testnet defaults, to replace literal Testnet variant.
• Replace matching against Testnet with an is_mainnet() method. (This is only required when we're matching against Testnet, we can change the Mainnet matches gradually.)

[mpguerra]

Lowering priority as it's not clear that it's a blocker for ZSA testing anymore

[arya2]

Elevating priority so ZSAs can be tested with a new genesis block.

[teor2345]

Elevating priority so ZSAs can be tested with a new genesis block.

Mining a new genesis block is out of scope for this ticket. It's an extremely tricky process, let's check what zcashd does, and open another ticket?

(It's possible they just re-use the mainnet or testnet block for all networks.)

[daira]

zcashd uses a separate genesis block for testnet, and does not support multiple testnets without modifying code and rebuilding. Is it sufficient to just use that block and upgrade at height 1?

[teor2345]

zcashd uses a separate genesis block for testnet, and does not support multiple testnets without modifying code and rebuilding. Is it sufficient to just use that block and upgrade at height 1?

@arya2 let's ask the QEDIT team when we get in touch with them? (Along with everything else, it's not an urgent question.)

[mpguerra]

@arya2 can you split out what we're going to work on from here on Sprint 24?