DEV: Add converter framework for migrations-tooling (#28540)

* Updates GitHub Actions
* Switches from `bundler/inline` to an optional group in the `Gemfile` because the previous solution didn't work well with rspec
* Adds the converter framework and tests
* Allows loading private converters (see README)
* Switches from multiple CLI tools to a single CLI
* Makes DB connections reusable and adds a new abstraction for the `IntermediateDB`
* `IntermediateDB` acts as an interface for IPC calls when a converter steps runs in parallel (forks). Only the main process writes to the DB.
* Includes a simple example implementation of a converter for now.

migrations/README.md

@@ -1,7 +1,31 @@
 # Migrations Tooling
 
+## Command line interface
+
+```bash
+./bin/cli help
+```
+
+## Converters
+
+Public converters are stored in `lib/converters/`.
+If you need to run a private converter, put its code into a subdirectory of `private/converters/`
+
 ## Development
 
+### Installing gems
+
+```bash
+bundle config set --local with migrations
+bundle install
+```
+
+### Updating gems
+
+```bash
+bundle update --group migrations
+```
+
 ### Running tests
 
 You need to execute `rspec` in the root of the project.