Add root metadata class to new TUF metadata model

[lukpueh]

Description of issue or feature request:

Add root metadata class to tuf.api.metadata module and implement (de)serialisation methods (to_dict and from_dict) and adequate metadata update methods (see tuf.api.Signed-subclasses for inspiration).

Current behavior:
No Root class in tuf.api.metadata

Expected behavior:

# In tuf/api/metdata.py
class Root(Signed):
    ...

[sechkova]

Apologies if I had ruined someone's first issue but I needed this so I opened #1193 😁

I want to mention that in the tests I had to fix dictionaries passed by reference and afterwards used as expected updated values.
For example:

        fileinfo = snapshot.signed.meta // by reference
        hashes = {'sha256': 'c2986576f5fdfd43944e2b19e775453b96748ec4fe2638a6d2f32f1310967095'}
        fileinfo['role1.json']['version'] = 2
        fileinfo['role1.json']['hashes'] = hashes
        fileinfo['role1.json']['length'] = 123

        snapshot.signed.update('role1', 2, 123, hashes) // does not do anything really
        self.assertEqual(snapshot.signed.meta, fileinfo)

I think we should design the metadata classes in a way that discourages such usage.
Maybe #1139 is where this can take place?

[joshuagl]

I want to mention that in the tests I had to fix dictionaries passed by reference and afterwards used as expected updated values.
For example:

        fileinfo = snapshot.signed.meta // by reference
        hashes = {'sha256': 'c2986576f5fdfd43944e2b19e775453b96748ec4fe2638a6d2f32f1310967095'}
        fileinfo['role1.json']['version'] = 2
        fileinfo['role1.json']['hashes'] = hashes
        fileinfo['role1.json']['length'] = 123

        snapshot.signed.update('role1', 2, 123, hashes) // does not do anything really
        self.assertEqual(snapshot.signed.meta, fileinfo)

I think we should design the metadata classes in a way that discourages such usage.
Maybe #1139 is where this can take place?

Good catch, thanks! That does seem like another good argument for using classes for metadata fields, rather than passing dicts around.

[joshuagl]

fat-fingered the wrong button 🤦