Added support for case management methods

Resolves: #144

Ref Documentation: here

Chronicle methods added:

• chronicle.case.list_cases()
• chronicle.case.get_case()
• chronicle.case.patch_case()
• chronicle.case.merge_cases()
• chronicle.case.execute_bulk_add_tag()
• chronicle.case.execute_bulk_assign()
• chronicle.case.execute_bulk_change_priority()
• chronicle.case.execute_bulk_change_stage()
• chronicle.case.execute_bulk_close()
• chronicle.case.execute_bulk_reopen()

CLI Commands:

• secops case list
• secops case get
• secops case update
• secops case merge
• secops case bulk-add-tag
• secops case bulk-assign
• secops case bulk-change-priority
• secops case bulk-change-stage
• secops case bulk-close
• secops case bulk-reopen

Testing:

• Added integration tests in: tests/chronicle/test_case_integration.py and tests/cli/test_case_integration.py
• Added unit tests in: tests/chronicle/test_case.py

Documentation:

• Updated README.md with Chronicle client usage examples
• Updated CLI.md with CLI command examples
• Updated api_module_mapping.md with v1beta endpoint mappings