Add GTFS Validator to CI pipeline

[mmathieum]

Links:

• https://gtfs.org/schedule/validate/#use-this-validator-as-part-of-your-data-pipeline
• https://github.com/MobilityData/gtfs-validator#run-the-app-via-command-line

[mmathieum]

https://github.com/mtransitapps/commons/commit/2b92e16c29cdf0b820287f5a4923bac92f75170b

[mmathieum]

https://github.com/mtransitapps/commons/commit/6f25b3f8f0fc103b82435c8b242d98d5fe542ac1

[mmathieum]

https://github.com/mtransitapps/commons/commit/0728e030bc8b1f306ff57cf6430751f2d3a7449e https://github.com/mtransitapps/commons/commit/f3dd95819b1138163e6b6765a9848825c7121b05 https://github.com/mtransitapps/commons/commit/b2a54a03c4c82e13d0541b24b94a9164866a2c0e