Allow user defined path to Sylvia crate

[jawoznia]

Currently we use the proc_macro_crate to find the Sylvia crate. This way the generated code can work even if an alias was used for the dependency.

pub(crate) fn crate_module() -> Path {
    use proc_macro_crate::{crate_name, FoundCrate};

    match crate_name("sylvia").expect("sylvia is not found in Cargo.toml") {
        FoundCrate::Itself => parse_quote!(sylvia),
        FoundCrate::Name(name) => {
            let ident = syn::Ident::new(&name, proc_macro2::Span::mixed_site());
            parse_quote!(#ident)
        }
    }
}

If however in the future a need for providing custom path to the Sylvia crate would occur we could consider adding serde-like functionality:

#[derive(Serialize, Deserialize, Clone, PartialEq, JsonSchema, Debug, Default)]
#[serde(crate = "sylvia::serde")]
pub struct AllAllowancesResponse {
    pub allowances: Vec<AllowanceInfo>,
}

This was also discussed here https://users.rust-lang.org/t/how-to-express-crate-path-in-procedural-macros/91274/17.