Feature: Add WMS/XYZ Layer

[srijitcoder]

What does this PR do (Ref - #234)

• Allow users to add external layers. Need to pass addExternalLayer: true in eox-layercontrol
• User can add WMS/XYZ layer URL directly to the layercontrol. They can search different layer and directly add it to the map.
• They can also import EOx-LayerJSON to the map.
• Used eox-map method called addOrUpdateLayer to add new layer to the layer's list