feat: validate mutually exclusive authentication fields for ssh_runner

[deepsikha-dash]

Description

• Added support for validating mutually exclusive authentication fields in plugin metadata.
• Extended ssh_runner metadata with password and private_key fields.
• Added a validation rule to ensure these authentication methods are mutually exclusive.
• Updated plugin_validator.py to validate the mutually_exclusive property and verify referenced field IDs.

Related Issues

Closes #848

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

Verified the changes using:

python scripts/validate_plugins.py --plugin ssh_runner
python scripts/refresh_plugin_checksum.py --plugin ssh_runner

Validation completed successfully and the plugin checksum was confirmed to be up to date.

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.

[utksh1]

Requesting changes. This adds a metadata-level mutually_exclusive declaration, but it does not add runtime input enforcement or tests proving ssh_runner rejects payloads that provide both password and private_key. Please add focused validator/runtime coverage for the mutual-exclusion contract before merge.