feat: sequential execution of middleware

[Ayush5120]

• Support for sequential execution of middleware to be injected into incoming POST /tasks requests
• Middleware classes to be executed can be specified, in fully qualified docstring notation, in the app configuration, in the desired order of execution, e.g.:

  
  middlewares:
  - "package.subpackage.module.MiddlewareClass1"
  - "package.subpackage.module.MiddlewareClass2"

• It is possible to specify one or more "fallback middlewares", in case the desired middleware fails; to achieve that, list nesting by one level is supported, such that each inner list is treated as a single "middleware group", from which at most one middleware is applied - the first one that does not raise a MiddlewareException; in the following example, proTES first attempts to apply MiddlewareClass1a; if it works, it will advance to the next item in the outer list, MiddlewareClass2 and try to apply that; however, if MiddlewareClass1a raises an error, proTES will first try to apply MiddlewareClass1b before progressing to MiddlewareClass2:

  middlewares:
  - - "package.subpackage.module.MiddlewareClass1a"
    - "package.subpackage.module.MiddlewareClass1b"
  - "package.subpackage.module.MiddlewareClass2"

• If all middlewares of any middleware group fail (including in groups of size 1, i.e, without list nesting), proTES will respond with a 500 response
• Developers can implement their own middlewares by implementing the pro_tes.middleware.abstract_middleware.AbstractMiddlewareClass; of course, any middleware class provided via external packages needs to be made available to the application (e.g., by adding the corresponding package to requirements.txt and rebuilding the app and worker images)
• Available middlewares for task distribution (random and distance-based) are now available in pro_tes.plugins.middlewares.task_distribution with all their dependencies
• Task distribution middlewares are applied by default, as a single middleware group, with the distance-based distribution (pro_tes.plugins.middlewares.task_distribution.distance.TaskDistributionDistance) being attempted first, and with the random-based task-distribution (pro_tes.plugins.middlewares.task_distribution.random.TaskDistributionRandom) serving as a fallback; this behavior can be changed by modifying the middlewares app configuration section
• Developers can optionally make use of the base class pro_tes.plugins.middlewares.task_distribution.base.TaskDistributionBaseClass for implementing other task distribution middlewares

Minor changes:

• Replace legacy (Python <=3.7) Type hints for Lists, Dicts and Tuples with their lowercase counterparts across the entire application
• Silence mypy complaints about missing type hints/stubs in external packages
• Drastically simplify middleware handler and task distribution middleware code
• Streamline POST /tasks controller code as a preparatory step for a major refactoring
• Streamline logging
• Temporarily disable unit tests in preparation for an effort to write consistent unit tests for the entire application

Closes #124

[Ayush5120]

I'm really sorry to be nagging, but I really want to get this right before it's copied into proWES :-S

No Problem at all, I will fix them all in the next commit.