[CDRIVER-5535] Create asset groups in Silk (#1619)

* Add a script and Earthly target to create a Silk asset group
* Define new EVG tasks:

- Add a "Miscellaneous" "variant" to group one-off tasks that don't vary.
- Add a "create-silk-asset-group" task that creates a Silk asset group
  for the current branch if it does not already exist.

* Ignore a `.secret` file in the repo directory
* Run the asset group creation on all builds