TAP 15 (succinct hashed bin delegations) implementation

[MVrachev]

Description of issue or feature request:
TAP 15 was created on June 23-rd 2020 and was last modified in July 6-th 2020.
Since then the TAP has been put to draft status meaning it needs a prototype implementation before it's accepted as a future specification change.
Given that this TAP underlines an efficient way of handling hash bin delegations, meaning it would be really useful when python-tuf is integrated into Warehouse, it's logical that we should not only create a prototype but directly work to integrate it in python-tuf.

The result I am expecting from this issue is:

1. Support reading, updating, and saving target metadata files that are using succinct hashed bin delegations.
2. An exemplary document inside examples/ showcasing how succinct hashed bin delegations can be utilized in practice.

I have come up with the following steps:

• Add and update the necessary attributes to support reading and writing from/to a target file with succinct hashed bin delegations
• Add simple testing for target files using succinct hashed bin delegations.
• Make sure the client parses the succinct hash bin delegated metadata
• Test that the client parses the succinct hash bin delegated metadata and visits the expected bin for a specific target
• Test actually downloading a target
• Add a script inside examples/
• Testing the examples/ script

[lukpueh]

This is exactly what I had in mind for this issue! Plus, the client also needs to be able to parse and interpret the new metadata, in order to find the correct bin for a target.

[MVrachev]

This is exactly what I had in mind for this issue! Plus, the client also needs to be able to parse and interpret the new metadata, in order to find the correct bin for a target.

Agree, added it to the TODO list.