Imagery definition refactor
Extend imagery definitions by:
- allowing setting default layers for WMS_ENDPOINT and WMTS
- allowing setting minimum expires time for tile for this imagery
- allowing setting custom headers that will be sent for all requests
(get map, get capabilities) for this imagery
Additional changes in code:
- use TileJobOptions to pass miscellaneous options to loaders
- refactor WMSImagery to use SAX parser
See: #15981, #7953, #16224, #15940, #16249