feat: add chain names & tx cache URL

[anna-carroll]

• add SignetEnvironmentConstants and SignetConstants (system + environment)
  • contains names for Host and Rollup networks & the Transaction Cache URL
• adds TxCache::pecorino() and TxCache::pecorino_with_client(client: Client)
• updates Filler and Order example code to use new constants

[anna-carroll]

request for style feedback

[prestwich]

I'd rather there be something like SignetEnvironmentConstants::tx_cache()

yeah, the way to achieve that is to fold the constants into a binary and have that binary's configs/constants object handle it. push relationships between types up to higher abstractions

[prestwich]

just a few doc nits