Key id must match in all circumstances

[selfsimilar]

Previously, the jwks() function in ProviderController.php would return a default kid value if the $crypt->kid value was null. This could be seen in the output of .well-known/jwks.json which would return

{"keys":
[{"alg":"RS256","kty":"RSA","use":"sig","kid":1,"n":"abced..."
}]}

This had two problems. Firstly, the kid needs a string value, not a numeric. Secondly, the user token issued by the POST /oauth/token route would have a kid value of null. Therefore, no clients could match the key needed to decode the JWT.

This fix allows for the kid to be set via the KeyRepository methods if necessary, and by default sets a default string kid value for the returned CryptKey object.