Add Ed25519 for signing / Include minisgn

[rugk]

Is it possible to include Minisgn (by @jedisct1) into this project or to add Ed25519 signing as a possible option (or even as the default if you want)?

Related: Ed25519 was already mentioned in #43 and in #119

[SantiagoTorres]

Hello, @rugk. I think ed25519 is already supported via #289, unless I'm missing additional context.

[rugk]

Okay, good to know.

[vladimir-v-diaz]

That's correct! Like Minisgn, we also depend on libsodium (via PyNaCl's bindings) to generate Ed25519 keys and signatures. We also verify Ed25519 signatures in pure Python with PyCA's Ed25519.

We'll look into Minisgn if we decide to allow external verification of TUF's embedded signatures in the future.

Thanks for the recommendation!