ogc: Support Additional WMS 1.3.0 URL Parameters

[benforshey]

Is your feature request related to a problem? Please describe.
Some of the URL parameters defined in the WMS 1.3.0 spec are not included in the ocg package. I recently tried to use this package to implement a WMS layer with a time-enabled WMS 1.3.0 service, but wasn't able to add the time URL parameter. Additionally, the srs URL parameter needed to be named crs for this particular WMS endpoint.

Describe the solution you'd like
I'd like to submit a PR that adds support for the following:

• referring to the reference system as either crs or srs. E.g., the URL parameter would support either &crs=EPSG:3857 or &srs=EPSG:3857.
• add exceptions as an optional URL parameter
• add time as an optional URL parameter
• add elevation as an optional URL parameter
• add a rest parameter in the WmsMapType function that allows users to send non-spec URL parameters / override existing URL parameters as needed.

Describe alternatives you've considered
I tried to append the URL parameters I wanted to use onto the end of the url property passed into the WmsMapType function. That function prepends ? to its list of URL parameters, so I could not do something like url: "https://www.mrlc.gov/geoserver/NLCD_Land_Cover/wms/?time=12345", or it would result in a final URL like https://www.mrlc.gov/geoserver/NLCD_Land_Cover/wms/?time=12345?bbox=12.34,56.78&....

Additional context
I really appreciate the work that's gone into this package—I wouldn't have even known where to start without it! If my proposed changes are something that might fit well with the ogc package, I'd be quite glad to put in the work to make this happen.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. Please comment here if it is still valid so that we can reprioritize. Thank you!

[benforshey]

Still willing to make the PR, if this is a good direction for the library.