Recommend to install the Dependency Analytics extension after opening a pom.xml (once)

[fbricon]

if they do not have the Dependency Analytics extension installed already, vscode-xml should offer users to install it once when they open a pom.xml.

See redhat-developer/vscode-java#1771.

vscode-java does it already, so we need a way to avoid displaying the same recommendation several times from several etensions.