Support loading Python processors from NARs

Currently, third-party dependencies for Python Processors can be handled in two ways. Either they can be declared as dependencies in a Processor itself; or the Processor can be in a module where a requirements.txt dictates the requirements. These can be very helpful for developing Python based Processors.

However, in production environments, it is not uncommon to see environments where pip is not installed. There is an inherent risk in allowing remote code to be downloaded in an ad-hoc manner like this, without any sort of vulnerability scanning, etc.

As such, we should allow users to also package python packages in NiFi's native archiving format (NARs).

The package structure should be as follows:

my-nar.nar
+-- META-INF/
    +-- MANIFEST.MF
+-- NAR-INF/
    +-- bundled-dependencies/
        +-- dependency1
        +-- dependency2
        +-- etc.
+-- MyProcessor.py

Where MyProcessor.py could also be a python module / directory.

In this way, we allow a Python Processor to be packaged up with its third party dependencies and dropped in the lib/ directory (or extensions) directory of a NiFi installation in the same way that a Java processor would be.