Make BurnchainParameters TOML-configurable.. possibly even PoxConstants

[gregorycoppola]

Is your feature request related to a problem? Please describe.
Right now, BurnchainParameters is not TOML configurable.
https://github.com/stacks-network/stacks-blockchain/blob/a0393e47452ae45eea645d792717d41035320961/src/burnchains/mod.rs#L98

This can be difficult for running a custom testnet (like for Stacks 2.1), because you might want to change first_block_height on the fly when trying out new test nets.

The same could arguably be said for PoxConstants, for which we recently wanted to set v1_unlock_height.

The call stacks for BurnchainParameters::from_params and PoxConstants::new are documented in #3411

Describe the solution you'd like
Make BurnchainParameters configurable in the TOML file, and maybe even PoxConstants.

Describe alternatives you've considered
The current way to configure BurnchainParameters is to change the Rust code.

Additional context
#3411

[jcnelson]

I'm already working on this in #3409

[jcnelson]

The parts of PoxConstants that need to be changeable are in #3409.

In general, there's no need to make BurnchainParameters configurable. Those are meant to be constants. If you need to spin up a dedicated testnet that doesn't talk to the main testnet, then you really should do so in a separate PR so we don't confuse users.

[blockstack-devops]

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