CLI: add command to update aggregator configuration

[divergentdave]

I just ran curl -i -XPATCH -H "Authorization: Bearer $DIVVIUP_TOKEN" -H 'Content-Type: application/vnd.divviup+json;version=0.1' -H 'Accept: application/vnd.divviup+json;version=0.1' --data '{}' "https://api.staging.divviup.org/api/aggregators/$AGGREGATOR_ID" to update feature flags of our aggregators. We currently have CLI subcommands to rename an aggregator or rotate an aggregator's bearer token, which would both re-fetch aggregator configuration as a side-effect, but it would be nice to provide a dedicated update/refresh command that just fetched the config.