RepositorySimulator: add support for delegated hash bins

Description of issue or feature request:

Currently repository_tool.py provides an easy way to distribute a large number of target files over multiple delegated roles (hashed bins). A feature described in PEP458 that would be used in warehouse. https://github.com/theupdateframework/python-tuf/blob/2206fc917e46ff584dcae75e640e1144c8b9f078/tuf/repository_tool.py#L2494

Having such functionality in RepositorySimulator that uses metadata api will be useful for testing the newer code.

[joshuagl]

The hashed bin delegation code in repository_tool was cleaned up and simplified when the Warehouse work kicked off. See, for example, https://github.com/theupdateframework/python-tuf/pull/1007 and https://github.com/theupdateframework/python-tuf/pull/1012. Specifically:

• delegate_hashed_bins()
• add_target_to_bin()
• remove_target_from_bin()

As a follow-on we refactored some of the helper functions to be public API (see https://github.com/theupdateframework/python-tuf/pull/1058) in repository_lib for use with an early iteration of the metadata API:

• create_bin_name()
• get_bin_numbers()
• find_bin_for_target_hash()
• get_target_hash()

[jku]

IMO this feature should not be implemented in test code but rather be part of repository tooling (#1136).

absolutely true as the mid/long term goal but it might still make sense to implement in RepositorySimulator first:

• no need to wait for the library module to be able to test ngclient
• gives us data about the complexity and what a good API (implemented on top of Metadata API) might look like