make ED25519PrivateKeyValue public, add a way to instantiate ed25519Signer from it

[arbll]

What does this PR do ?

1. Makes Ed25519PrivateKeyValue public
2. Adds a way to instantiate ed25519Signer from Ed25519PrivateKeyValue

With the change of interface for the keys there is no non-hacky way to use an ed25519 key that is not stored as a go-tuf specific marshaled JSON blob without re-implementing the signer interface.

[coveralls]

Pull Request Test Coverage Report for Build 1758509545

• 2 of 10 (20.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 70.068%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/keys/ed25519.go	2	10	20.0%

Totals
Change from base Build 1752556678:	-0.2%
Covered Lines:	2156
Relevant Lines:	3077

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💛 - Coveralls

[trishankatdatadog]

@hosseinsia @cedricvanrompay-datadog

[asraa]

Nice! I think this is totally valid for a use case where you've done offline key generation.

Generally speaking, do we want some kind of import naming/method/something else? I'm happy with this, maybe the CLI should also allow for key imports at some point.