gcsfuse_mount_spec parsing and supporting mount_options from command line

[deepikarajani24]

This PR improves the command-line parsing for --gcsfuse_mount_spec to allow for multiple key-value pairs and robustly handle custom GCS FUSE mount_options.

Usage

1. To specify custom mount_options:
   The entire argument must be wrapped in single quotes ('...') to protect the inner double quotes from the shell.

--gcsfuse_mount_spec='gcs_path=gs://my-bucket/data,mount_path=/data,mount_options="implicit-dirs,metadata-cache-ttl-secs=-1"'

2. To use default mount_options:
   No special quoting is needed.

--gcsfuse_mount_spec=gcs_path=gs://my-bucket/data,mount_path=/data

Testing
Unit Tests: Added parameterized unit tests to validate the new parsing logic for various formats (complex, basic, and empty mount_options).

End-to-End Test: Confirmed the full functionality by successfully launching a Fuji 7B training job that used a custom GCS FUSE mount defined with this flag.

The existing mount flags hardcoded in the code are not changed and I do see errors with those but haven't changed them. Instead passing the mount options I need in the command.