Add required key protection to AuthenticatorSelectionCriteria

[gmandyam]

Regarding https://w3c.github.io/webauthn/#authenticatorSelection, add criterion for required key protection.

Suggestion is to define
enum KeyProtection {
"SW",
"HW",
"TEE",
"SE"}

If requireResidentKey=false, then this criterion will be ignored. If requireResidentKey=true, then the UA will raise exception if the key protection level does not meet level specified.

[jyasskin]

For the web, we'll probably want to spell out the acronyms. What do they stand for?

I suspect these are still relevant for requireResidentKey==false: they'd describe how the encrypt-to-self key is protected.

[gmandyam]

@jyasskin

"SW" = software protection, "HW" = hardware protection, "TEE" = trusted execution environment, "SE" = secure element

[rlin1]

I suggest adding such changes to branch authnr-sel-addtl-fields and the related PR #442

[rlin1]

See branch authnr-sel-keyprotection

[nadalin]

Closing for now as there is no interest