Add methods to retrieve list of valid values

[Ryandaydev]

Retrieve a list of valid values for these parms:

Year Origination City Abbreviation Destination City Abbrevation

Implementation-wise, they could be select distinct queries.

[Ryandaydev]

These endpoints make sense from a flow as follows:

• Get list of valid city appreviations
• Get list of valid years
• Submit request using these values

For instance, a web site might populate drop downs with the valid values, then submit the request using those values.