Allow `fudge::companion` struct to be generic

[lemunozm]

Support generics in fudge::companion structs, as follows:

#[fudge::companion]
pub struct TestEnv <T> {
    #[fudge::relaychain]
    pub relay: RelaychainBuilder<T, ...>,

    #[fudge::parachain(2000)]
    pub parachain: ParachainBuilder<T, ...>,
}

This allows to create different TestEnv for different runtimes easier.