Determine which credential types are allowed for an adaptor

[sverhoeven]

For Xenon CLI it would be nice to only show the credential options that are allowed for a certain adaptor.

For example the KeyTabCredential is only allowed by the HDFS adaptor, showing the --keytab argument only when HFDS has been selected will make the CLI cleaner for the other adaptors.

[jmaassen]

Agreed. But how do we return these ? As strings ? Classes ?

[sverhoeven]

I prefer classes as these are less typo prone.

[jmaassen]

Sure, no problem. I'll add a getSupportedCredentials to AdaptorDescription