feat: allow string interpolation on resource options

[chesedo]

This PR allows options passed to resources to be string interpolated. For example in this local_uri option, the password is being read from the Secrets.toml file

#[shuttle_service::main]
async fn main(
    #[shuttle_shared_db::Postgres(
        local_uri = "postgres://postgres:{secrets.password}@localhost:16695/postgres"
    )]
    pool: PgPool,
) -> shuttle_service::ShuttlePoem<impl poem::Endpoint> {
    ...
}

With #596, this closes #497

[oddgrd]

This is really cool! 🤩 This may be obvious, but what will the error be if it tries to interpolate a secret and it doesn't exist? It looks like it will always be interpolation error, should there also be a not found error if we check and it doesn't exist? Or we could just add a bit about making sure the secrets is added to Secrets.toml to the interpolation error message..

[chesedo]

@oddgrd This is the error message:

Error: Failed to interpolate string: KeyError(Invalid key: secrets.passwor)

[chesedo]

Now the error is 😄

Error: Failed to interpolate string. Is your Secrets.toml correct?

Caused by:
    KeyError(Invalid key: secrets.Password)