Add `AlchemiscaleClient` methods `set_tasks` and `set_transformations_tasks`

[dotsdl]

There are users and downstream projects, such as asap-alchemy, that are more interested in ensuring that a certain number of Tasks exist per Transformation than in creating new Tasks.

This is especially the case when a user creates many iterations of AlchemicalNetworks in the same Scope that feature many Transformations in common. In this scenario, simply calling create_tasks or create_transformations_tasks on every Transformation of the submitted AlchemicalNetwork may generate more work than is necessary given existing results. A user that wants to only create new Tasks for entirely new Transformations would need to do a number of client calls to determine which Transformations have no Tasks before creating new ones.

To address this, we should create two new methods on the AlchemiscaleClient:

• set_tasks: set the minimum number of Tasks on the given Transformation (and optionally extendsing another Task)
• set_transformations_tasks: set the minimum number of Tasks for multiple Transformation (and optionally extendsing another Task), in a single call

These methods are functionally idempotent versions of create_tasks and create_transformations_tasks that already exist in the AlchemiscaleClient. They should take the same inputs as these existing methods, but will only create new Tasks where an insufficient number already exist.

[dotsdl]

CC @JenkeScheen