Add keytab support for SPNEGO apps

[smhmhmd]

As per this discussion, add keytab support.

It looks possible to extend credentials-fetcher to manipulate keytab files.
Keytab files can be shared with containers just like Kerberos tickets are being shared as of now.
This can be done with non-gMSA accounts as well, as long as password expiry interval is known.
If the password expiry interval is longer than keytab file refresh, this would work fine.

[zyloid]

+1

[smhmhmd]

Thanks @zyloid