Cannot have two or more static vaadin portlets

[krissvaa]

Cannot have two or more Vaadin static portlets because of that PORTLET_PARAMETER_STATIC_RESOURCES_MAPPING(portlet.static.resources.mapping) is system property with no option to use different mappings for each portlet.
For example: Migrating from version 14 to 23 it would be benefical two have both version static portlets for gradual upgrade.

One fix would be to enable having opetion for each portlet having option to use default (system property) or their own property defined in flow-build-info.json file.