New API: Add a constants module

[MVrachev]

Description of issue or feature request:
I think it will be useful to have a constant module in the new API.
This will help us to minimize the errors when writing hardcoded strings.
For example, the "snapshot.json" string is used often in the Timestamp class in tuf/api/metadata.py.

@joshuagl raised awareness for that in this comment:
#1223 (comment)

Current behavior:
Harcoded strings.

Expected behavior:
Use constant containing the string.

[jku]

For the specific case mentioned, could it not be Snapshot.filename in metadata.py as the filename seems to be a specified feature of snapshot metadata? That Snapshot.filename could be a "getter-only" property to underline how it's not supposed to be changed...

EDIT: actually the property might not be a good idea as you can't have class properties... Snapshot.filename (not instantiating Snapshot) only works if filename is a plain class variable. But that's Python: could make it Snapshot.FILENAME to make it look more constant

[MVrachev]

Do we need a separate module other than the one added in #1470?

[MVrachev]

@jku any opinions?

[MVrachev]

The Metadata API and the updater implementation have evolved sufficiently that we don't need one central place for constants.
Currently, we write the constants directly in the metadata API module itself and we have a separate one inside the updater here.
If in the future we decide we need one central place or to minimize the length of Metadata API we will create other issues for that.