Add functions to generate a similar key

[jschlyter]

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A common use-case when implementing rekeying or testing is to be able to generate a similar key to an already existing one. This PR adds a JWK.generate_similar(key) classmethod to support this, as well as a JWK.get_generate_params() to get the parameters used to generate a key. Test cases included.

Perhaps there's an easier way to do this; if so please enlighten me.

[simo5]

Looks like a reasonable approach

[simo5]

Please squash the fix in the proper patches and force push the result