New API: Don't hardcode metadata files extension

[MVrachev]

Description of issue or feature request:
In a review of a pr adding additional classes in the metadata API, @sechkova noticed that we have hardcoded
the usage of JSON files representing our metadata files after the implementation of abstract (de)serializer here.

For context: Teodoras comment

Current behavior:
Hardcoded *.json file names in a lot of places: https://github.com/theupdateframework/tuf/blob/52207862008cc54db8a43e1543a653be0d0333f7/tuf/api/metadata.py#L468

Expected behavior:
Don't hardcode the file names representing our metadata files with their file extensions.

TODO:

• Remove hardcoded json strings from the code in all classes.
• Fix our documentation to not give examples with JSON files or have a sentence like For JSON metadata files, here is how it will look like...
• Add support for other metadata file types inside the metadata classes.
  This could be achieved with a configuration option in a Repository class or helper functions for each of the metadata classes.

[MVrachev]

Added commit 1572385 which removed part of the places where we used
hardcoded JSON string and added support for other file extensions in the Timestamp` class.