Mechanism to let the plugins decide of the key order

[ewjoachim]

Hi!

I have exactly the same issue as str4d/age-plugin-yubikey#178 and olastor/age-plugin-fido2-hmac#25, explained in str4d/age-plugin-yubikey#178 (comment)

It looks like it boils down to solving the following TODO in the code:

age/plugin/plugin.go

Lines 417 to 418 in 2214a55

	// TODO: there should be a mechanism to let the plugin decide the order
	// in which identities are tried.

I could not find any issue/PR/discussion about actually solving this so I wanted to open the discussion to signal user interest in this being solved.