Replace "protobuf-dynamic" lib with built-in protobuf methods

[amaechler]

Description

The inline and file-based protobuf decoders depended on an external library protobuf-dynamic. Initially, I thought the library (which appears to be no longer actively maintained) doesn't support maps at all, but upon further testing, I was unable to confirm this.

However, instead of using a third-party lib, we can achieve the same functionality (building a Protobuf descriptor from a file or Base64-encoded string) using the standard protobuf library. This PR does so.

Additionally, this PR improves the code generation used in unit tests a lot:

• Generate the proto wrapper files as part of the build (using https://github.com/ascopes/protobuf-maven-plugin). This allows us to remove previously generated files and also clean up the checkstyle exclusions.
• Add documentation on how to generate individual descriptor files (which some tests use) and add a Dockerfile providing a container to generate those files.

---

This PR has:

• been self-reviewed.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever it would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• been tested in a test Druid cluster.

[amaechler]

Copied from our docs.

[amaechler]

Just added a few comments to this file (and made the constructor a bit cleaner) while reading through this file to understand what is happening.