ogc: Support Additional WMS 1.3.0 URL Parameters

**Is your feature request related to a problem? Please describe.**
Some of the URL parameters defined in the [WMS 1.3.0 spec](https://www.ogc.org/standards/wms) 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.