libtuf.py needs method for dynamically retrieving a delegated targets role

[trishankkarthik]

Observed

Presently, libtuf.py assumes that the developer knows the name of a delegated targets role before runtime. For example, retrieving the "unclaimed" role would be specified as repository.targets.unclaimed.

However, this is not always true or even possible. For example, how would a developer address in this manner a delegated targets role with the name recently-unclaimed? Furthermore, sometimes the developer may not known the names of the delegated targets role until name (perhaps because it is supplied by a user at the command-line).

Expected

We simply need a method for retrieving a delegated targets role by its name in a string. I will defer the exact mechanism to @vladimir-v-diaz , but a simple function should do the job.

[vladimir-v-diaz]

Delegated roles are no longer added as attributes of a Targets object by libtuf.py (e.g., repository.targets.delegated_role). The previous bahavior restricted rolenames to Python identifiers (i.e., can only include letters, numbers, the underscore character, and must start with a nonnumeric character). Now, delegated roles may be referenced as strings (e.g., repository.targets('recently-claimed')) and include characters other than '_'. In addition, methods have been added to return all the delegated rolesnames of a target (e.g., repository.targets.get_delegated_rolenames()) and the immediate delegated Target objects of a role.

Previous behavior: repository.targets.unclaimed.django.version = 8
Current behavior: repository.targets('unclaimed')('django').version = 8.

[trishankkarthik]

👍

Seems more Python-ish, though, to make it index-able (like dictionaries or lists). Community can decide this later.