Python HDFS: add Kerberos authentication support

[damccorm]

The HDFS client used by Beam supports Kerberos.

Initial idea: add a flag --hdfs_client that defaults to "INSECURE" and also accepts "KERBEROS". This flag will control initialization of self._hdfs_client.

HDFS client docs:
https://hdfscli.readthedocs.io/en/latest/api.html#module-hdfs.ext.kerberos

The HDFS client seems to use this Kerberos library:
https://pypi.org/project/requests-kerberos/

Imported from Jira BEAM-11750. Original Jira may contain additional context.
Reported by: udim.

[blazingbhavneek]

Hey there! 👋 I'm new to this repository and eager to contribute! 🌟 Could you kindly suggest some entry point or files to look into?

[damccorm]

Hey, saw you added this comment several places. I'd recommend focusing on a single issue at first (I answered the underlying question here - #20298 (comment))