feat: new is_official tag for feeds

[lawrencefan506]

Adding a new optional tag called 'is_official' for feeds as requested in issue #356.

Changes:

• Updated schemas in schemas/gtfs_realtime_source_schema.json and schemas/gtfs_schedule_source_schema.json to include this tag.
• Updated `tools/constants.py' to include this tag.
• Updated tools/representations.py, adding this tag to the Source class.
• Created, in tools/representations.py, a has_is_official method for the GtfsScheduleSource and GtfsRealtimeSource classes and a get_sources_by_is_official method for the SourcesCatalog class.
• Created test cases in tools/tests/test_representations.py for each new method listed above. All passed locally.
• Created get_sources_by_is_official method in tools/operations.py.
• Created test case in tools/tests/test_operations.py for the new method listed above. Passed locally.
• Updated README.md schemas to include this tag.

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