feat: JLink plugin

[vpa1977]

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This PR adds jlink plugin that allows to generate Java runtime tailored for the Java application.
When ran without supplying jar file property it will search the staging area for any jars and generate an image for them.
If no jar files are found, then java.base image will be generated.

This PR includes unit and integration tests.
The plugin documentation is split into separate PR: #689

Sample ROCK using plugin: https://github.com/canonical/openjdk-rocks/tree/prototype/demos/java-runtime

[lengau]

This is probably worth having a plugin-jlink spread test like the plugin-python ones.

[vpa1977]

Added PR for the linkcheck linter failure: #700

[vpa1977]

This is probably worth having a plugin-jlink spread test like the plugin-python ones.

Thanks!!! I have added one.

[lengau]

I discussed this with my team this week, and the conclusion was that the way we should do this is as an option for the base Java plugin.

So the default behaviour would be the current behaviour (which I think is to not stage a JRE?), but the jlink options here would be changed to <plugin>-jlink-<option>, and if any of them are set, it would use jlink to stage the JRE. Do you think that's reasonable/feasible?

[lengau]

Under the suggested change about putting this into the java plugins, this would be something we could add in get_build_environment (which is still override-able by users if need be)

[vpa1977]

I discussed this with my team this week, and the conclusion was that the way we should do this is as an option for the base Java plugin.

So the default behaviour would be the current behaviour (which I think is to not stage a JRE?), but the jlink options here would be changed to <plugin>-jlink-<option>, and if any of them are set, it would use jlink to stage the JRE. Do you think that's reasonable/feasible?

Hmm, so we will have to enumerate plugin properties in java plugin and retrieve ones with jlink key in it?

Also we need ability to decouple build and stage Java.
Example is test libraries requiring higher version then the release jar.
In this case we will have to stage Java runtime in a separate part.