Add classes for complex metadata fields

[lukpueh]

Coordinate with #1133 and #1137

Description of issue or feature request:
For consistent use of OOP we should provide classes for all complex metadata fields
and not only for the outer metadata signature wrapper and the inner signed metadata.

This would make the used metadata format more explicit and might have a positive effect on metadata schema validation (see discussion in #1130).

Current behavior:
Mixed use of classes and dictionaries for TUF metadata

Expected behavior:
Provide classes for all complex metadata fields, i.e. 'signatures' (in
Metadata) 'meta'/'targets' (in Timestamp, Snapshot, Targets), 'delegations'
(in Targets), 'keys'/'roles' (in not yet existent Delegation), ...