Add extensions for Gradle Quarkus projects

[xorye]

Fixes #112

To test, create a Gradle project using the Quarkus Maven plugin as explained in the guide:

mvn io.quarkus:quarkus-maven-plugin:0.28.1:create \
    -DprojectGroupId=my-groupId \
    -DprojectArtifactId=my-artifactId \
    -DprojectVersion=my-version \
    -DclassName="org.my.group.MyResource" \
    -Dextensions="resteasy-jsonb" \
    -DbuildTool=gradle

Open the Gradle project in vscode
Call the Add extensions to current project command in the command palette
Choose extension(s). (I'm not sure if multiple extensions will work yet, they've made the fix recently: quarkusio/quarkus#5224)
The command to add extension(s) should run in the terminal.

If Gradle wrapper exists, Gradle wrapper will be used for the command.

I have 13 commits for the PR for now, I will squash them into one when we are sure everything works.

I did not test this on Windows yet.

[fbricon]

Works for me. Except for the multiple extension part. But it still works in Maven, so that's fine.
The sad thing is the fix for adding multiple extensions is not part of the 1.0.0.CR1 release, so we'll see it working in a couple weeks only. But no need to try to workaround it until then.

Just want to make sure it works in Windows then I'll be ok to merge.

[xorye]

@fbricon its working on Windows