feat: impl FromStr for chain constants

[anna-carroll]

in service of implementing FromEnvVar in bin-base using impl_for_parseable! macro

Implemented for all types with pecorino()/test() instantiations, except purposefully did not implement for TxCache because I thought we may want the behavior for that to be different - e.g. treat the env var as the tx cache endpoint.

[prestwich]

if you want this to be even more rust idiomatic:

pub enum KnownChains { 
    Pecorino,
    #[cfg(...)]
    Test,
}

impl FromStr for KnownChains {
     type Err = ParseChainError;

     fn from_str(&self) { 
        // do chain name parsing
        // and the match block repeated across your existing impls
        // other impls use `let chain: KnownChain = s.parse()?;`
    } 
}