Regroup internal EODAG params in a constants file

[bjgiraudon]

Is your feature request related to a problem? Please describe.
When developing on EODAG, parameters names, such as "missionStartDate" are usually hard-typed string, repeated in many places of the code base. Using a constant file would reduce the risk of typo happening when trying to access a given parameter, which value is constant.

Describe the solution you'd like
We could have something like an eodag internal constants file, listing the parameters that are used in EODAG and constant.
The code would import the constants from there.