[WIP] Add new goal add-toolchain for adding new toolchains dynamically

[Syquel]

This is a Work-In-Progress to gather some feedback.
Integration tests will be added.

Currently Maven toolchains must be configured manually or in other ways.
Especially in ad-hoc build environments like GitHub Actions this requires developers to write Shell scripts or other logic in the pipeline to create the toolchain.xml .

By providing this goal developers would be enabled to execute tests with multiple JDKs without having to run the build multiple times (or as a matrix build) in such ad-hoc build environments.

[rfscholte]

I think this is a very ambitious plan (probably too ambitious).
Keep in mind that order should be preserved, just like comments.

[mthmulders]

If it's specifically for GitHub actions, please know that since Maven 3.6.2, Maven will replace environment variables in toolchains.xml. This allows you to setup a toolchains.xml using the environment variables that GitHub Actions give you. An example is this workflow.

[linux-china]

How about download JDK from https://adoptium.net/ or https://api.foojay.io/swagger-ui automatically? It's will be very useful for local development. Now I built a CLI to manage JDK versions in toolchains.xml https://github.com/linux-china/maven-toolchains-cli

[gnodet]

I think this is superseded by #14