Add ability to set extension capability from gradle plugin

[glefloch]

This adds the ability to set capabilities from within the gradle extension plugin.

Capabilities can be used as such:

quarkusExtension {
  capabilities {
    capability 'org.acme:ext-z'
    capability 'org.acme:ext-a' onlyIf ['org.acme:ext-b', 'org.acme:ext-y']
    capability 'org.acme:ext-c' onlyIfNot ['org.acme:ext-d']
    capability 'org.acme:ext-g' onlyIf ['org.acme:ext-b'] onlyIfNot ['org.acme:ext-d']
  }
}

[quarkus-bot]

This workflow status is outdated as a new workflow run has been triggered.

Failing Jobs - Building cc2bd0e

Status	Name	Step	Failures	Logs	Raw logs
✖	Initial JDK 11 Build	Build	Failures	Logs	Raw logs

Failures

⚙️ Initial JDK 11 Build #

- Failing: devtools/gradle/gradle-extension-plugin 
! Skipped: integration-tests/gradle 

📦 devtools/gradle/gradle-extension-plugin

✖ Failed to execute goal net.revelc.code:impsort-maven-plugin:1.6.2:check (check-imports) on project io.quarkus.extension.gradle.plugin: Imports are not sorted in /home/runner/work/quarkus/quarkus/devtools/gradle/gradle-extension-plugin/src/main/java/io/quarkus/extension/gradle/QuarkusExtensionConfiguration.java