Add support for signet

[obycode]

We would like to be able to run a stacks-node against a Bitcoin Signet network.

Known tasks:

• [ ] Adding a some constants in stackslib/src/burnchains/bitcoin/indexer.rs
• [ ] Adding a bitcoin_signet function in stackslib/src/burnchains/mod.rs
• [ ] Adding signet support in testnet/stacks-node/src/config.rs

[wileyj]

cc @zone117x

[wileyj]

@obycode my biggest ask here is that we use normalized names for the network, i.e. signet vs noble gas names

[zone117x]

Also maybe useful to start versioning like we've discussed. Something like signet-1

[wileyj]

after speaking to jude about this, i get the sense that it would be quite a large change to the stacks-core codebase, some of it quite complex.

i think we should keep this open, but move it to icebox as future work since i don't see this happening anytime soon.

we do have viable testnet options using bitcoin regtest as a burnchain for now (particularly when combined with https://github.com/stacks-network/bitcoin-p2p-firewall to prevent blockstorms/reorgs while allowing external participants to sync both the burnchain and the stacks testnet.

[wileyj]

moving this to icebox for now since it's a pretty substantial amount of changes