Expose ways to integrate new key types better

[lestrrat]

Addresses #912

[codecov]

Codecov Report

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Additional details and impacted files

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##             develop/v2     #969   +/-   ##
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  Coverage              ?   71.93%           
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  Files                 ?       95           
  Lines                 ?    13855           
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  Hits                  ?     9967           
  Misses                ?     3054           
  Partials              ?      834           

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[lestrrat]

I don't like the API that exposes the hooks for the raw keys. I need to rethink it

[github-actions]

This PR is stale because it has been open 14 days with no activity. Remove stale label or comment or this will be closed in 14 days.

[lestrrat]

This isn't going to fly. Need to remove

[lestrrat]

Notes: need to fix x25519 for > go1.20

[lestrrat]

note: if we want to use crypto/ecdh for go >= 1.20. we're going to have to break back compat for x25519.Public/PrivateKeys, as ecdh.PublicKey/ecdh.PublicKey expects to be treated as pointers, whereas previously we used []byte, which meant that we didn't need to carry pointers. Aligning this with crypto/ecdh probably means we need to release v3