Centrally define sensitivity classifications for references to Elytron capabilities.

Predominantly the following 6 items are referenced across the application server: -

Authentication Context
Credential Store
HTTP Authentication Factory
SASL Authentication Factory
SSLContext
Security Domain

We should probably represent these as: -

(Authentication Context) -> Authentication Client Ref
(Credential Store) -> Credential
(HTTP / SASL Authentication Factory) -> Authentication Factory Ref
(Security Domain) -> Security Domain Ref
(SSL Context) -> SSL Ref

We already have 'Credential' defined so we can re-use that.