feat: add OAuth2 bearer token authentication support

[stevefulme1]

Summary

Add api_bearer_token as an alternative authentication method alongside existing API key pair authentication. When a bearer token is provided, modules use Authorization: Bearer <token> instead of HTTP Signature auth, and the cryptography Python library is not required.

Motivation

This enables OAuth2/OIDC workload identity flows where automation platforms (e.g., Ansible Automation Platform) issue short-lived JWT tokens that Intersight validates as OAuth2 bearer tokens. This supports:

• Zero-trust deployments — no static API keys stored in automation platforms
• Sovereign cloud compliance — short-lived credentials meet regulatory requirements
• Secure AI factory infrastructure — workload identity for automated provisioning

Changes

• plugins/module_utils/intersight.py: Add api_bearer_token to intersight_argument_spec, branch IntersightModule.__init__ to skip key loading for bearer auth, branch intersight_call to use Bearer authorization header
• plugins/doc_fragments/intersight.py: Document new api_bearer_token parameter with conditional requirement notes
• tests/unit/test_bearer_token_auth.py: 5 new unit tests covering bearer token auth path, validation, and backward compatibility
• tests/unit/test_v3_api_keys.py: Update existing test for new validation flow

Usage

# Bearer token auth (new)
- cisco.intersight.intersight_info:
    api_bearer_token: "{{ oauth2_token }}"
    server_names:
      - myserver

# Or via environment variable
# export INTERSIGHT_API_BEARER_TOKEN=<token>

# Existing key-based auth continues to work unchanged
- cisco.intersight.intersight_info:
    api_key_id: "{{ api_key_id }}"
    api_private_key: "{{ api_private_key }}"
    server_names:
      - myserver

Backward Compatibility

• All existing playbooks using api_key_id + api_private_key work unchanged
• api_key_id and api_private_key are now conditionally required (only when api_bearer_token is not provided)
• Clear error messages guide users when neither auth method is configured
• Zero changes to any of the 131 module files — auth is fully centralized