feat: Adds Gzip Processor and Content Inspector

[jshlbrd]

Description

This PR adds two new features:

• Gzip processor, to be used for converting data to and from Gzip
  • This is the first "data conversion" processor we've built, more detail on why is mentioned below
• Content inspector, to be used for checking the content (MIME) type of data
  • This accompanies the Gzip processor and should be used before applying data conversion processors

Motivation and Context

Substation assumes that all data coming into and going out of a pipeline is JSON, but there are use cases for converting bodies of data to and from different formats. This PR adds support for Gzip and inspection of any MIME type, but the design should be reused in future "data conversion" processors. These could include:

• encoded data (base64, hex, etc.)
• compressed data (zip, zlib, etc.)
• formatted data (CSV, XML, etc.)

For processor options, the simplest way to describe conversion is with an option called direction that accepts one of two strings: "to" and "from". This should be applicable to any type of data conversion (e.g., "from base64", "to zlib", "from CSV", "to XML") and I think it's better than having bespoke options ("decode base64", "compress zlib", etc.), but I'm open to alternative suggestions.

How Has This Been Tested?

Added a bunch of unit tests:

• Gzip processor has unit tests for every option
• Byter and Channeler factories have a unit test for the Gzip processor options
• Content inspector has unit tests for every option (success / fail of content type, with negation)
• Inspector factory has a unit test for the Content inspector options

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[julieagnessparks]

Looks good to me. Great commenting added for the additional functionality :)