provide an option to use `openssl` to sign instead of `ring`

[Leavrth]

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In order to comply FIPS 140, I need to ensure that all encryption uses the openssl FIPS dynamic link library. Would you be open to the idea of providing an option (feature?) to use openssl to sign here: https://github.com/EmbarkStudios/tame-oauth/blob/main/Cargo.toml#L32

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