Contribute to completion, hover .. with external JAR

[angelozerr]

Liferay is working on LSP4XML extension.

maven extension eclipse-lemminx/lemminx#166 is an another LSP4XML extension.

To support those extensions, vscode-xml should provide a mechanism to declare in the package JSON of the project the JAR which contains extension registered with SPI:

"contributes": {
		"xml.javaExtensions": [
			"./jar/org.eclipse.lsp4xml.extensions.maven-0.9.1.jar"
		]
}

When server is started it loop for each JAR to include it in the classpath (like vscode-java does).