Move clients, converters from iTEM Open Data repo

[khaeru]

See https://github.com/transportenergy/database/tree/main/item/historical

These concern some data flows from providers:

• ITF-OECD (cf. ITF-OECD community#15)
• UNECE (cf. UNECE community#8)
• OICA (cf. OICA community#11) done in Add module for OICA #13

There are also some un-merged PRs that contain data processing code for certain sources/flows:

• Add T015–T017 to clean three ADB ATO data sets transportenergy/database#78
• Add T018–T020 to clean three ADB ATO data sets transportenergy/database#80
• Add T011–T014 to clean 4 ADB ATO data sets transportenergy/database#81
• Add T021–T023 to clean 3 ADB ATO data sets transportenergy/database#83