feat: allow specifying https protocol for GMS connection

[timcosta]

This PR adds a new environment variable, DATAHUB_GMS_PROTOCOL, which allows you to specify either http or https for the connection to GMS. It defaults to http everywhere DATAHUB_GMS_HOST was set/used before, and a brief read of the code doesn't reveal any incompatibilities with this strategy.

One callout is that this is slightly different from the Java apps which use DATAHUB_GMS_USE_SSL as their env var, however there wasn't a good way to use a boolean for this use case as it's being parsed by pyyaml and there's no way to create an inline conditional.

[github-actions[bot]]

Unit Test Results (build & test)

59 tests  ±0   59 :heavy_check_mark: ±0   2s :stopwatch: -1s   1 suites ±0     0 :zzz: ±0    1 files   ±0     0 :x: ±0 

Results for commit 81da9cb0. ± Comparison against base commit fb61bd46.