Improve Gradle build

[jagedn]

Following https://github.com/nextflow-io/nf-nomad/issues/5#issuecomment-1956775991

this PR remove custom tasks and move the logic to a Gradle custom plugin

now you have following tasks:

• generateIdx, scan sources and generate extension.idx file to avoid add artifacts manually
• zipPlugin, compile and generate the zip
• unzipPlugin, after zipPlugin it extract the plugin into $HOME/.nextflow/plugin dir
• jsonPlugin, create the json required to create a release

unzipPlugin is util when you want to test in local without the need to use the NXF_PLUGINS_DIR