Add a crypto agility layer

[bifurcation]

This largely relies on go-hpke, with a few added nuances for MLS (signature, specifics of derivations).

[coveralls]

Pull Request Test Coverage Report for Build 633943037

• 125 of 203 (61.58%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+61.6%) to 61.576%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crypto.go	125	203	61.58%
<!--	Total:	125	203

Totals
Change from base Build 630150655:	61.6%
Covered Lines:	125
Relevant Lines:	203

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💛 - Coveralls