Provide a Progress support API for lemminx extension

[angelozerr]

Provide a Progress support API for lemminx extension

Fixes #1562

[angelozerr]

It should be nice to use this progress support inside lemminx (ex : show a progress bar to notify that XSD is downloading).

Today this support is used for lemminx-maven eclipse/lemminx-maven#473

Here how lemminx maven consumes the API https://github.com/eclipse/lemminx-maven/pull/473/files#diff-4a374a968fa37a849d87ec8835bd2a5acd917e74f36992a6866ff24a4e9106c7R244

Basicly:

1. get progress support

//  you get the progress support in the start of lemminx extension by using regsitry 
this.progressSupport = registry.getProgressSupport();

See https://github.com/eclipse/lemminx-maven/pull/473/files#diff-4a374a968fa37a849d87ec8835bd2a5acd917e74f36992a6866ff24a4e9106c7R183

2. consume progress support

ProgressMonitor progressMonitor = progressSupport != null ? progressSupport.createProgressMonitor() : null;
//  progressSupport.createProgressMonitor()  return toonull if LSP client cannot support progress

if (progressMonitor != null) {
	progressMonitor.begin("Loading Maven components...", "", 100, null);
}

if (progressMonitor != null) {
	progressMonitor.report("Initializing Plexus container" + getStepMessage(currentStep, nbSteps) + "...", percentage, null);
}

// another call of progressMonitor.report(

...			

progressMonitor.end();

See sample at https://github.com/eclipse/lemminx-maven/pull/473/files#diff-4a374a968fa37a849d87ec8835bd2a5acd917e74f36992a6866ff24a4e9106c7R244

[JessicaJHee]

LGTM! Works well when tested with lemminx-maven

[angelozerr]

Thanks so much @JessicaJHee for your review.