add configuration setting for forcing h2c

[edwardsb]

When using Fleet on GCP via Cloud run you run into a limitation attempting to upload software packages

Request URL:
https://dogfoodgcp.fleetdm.com/api/latest/fleet/software/package
Request Method:
POST
Status Code:
413 Payload Too Large

https://cloud.google.com/run/quotas#request_limits

32 MiB if using HTTP/1 server. No limit if using HTTP/2 server.

Which means you can only upload packages that are <= 32 MiB.

However, if using HTTP/2 there is no limit. Typically HTTP/2 is only enabled in the Go webserver if using TLS, but we can force HTTP/2 via H2C and the associated packages in the X lib.

After pushing the docker image with the new server configuration and enabling H2C in Cloud Run as well as setting FLEET_SERVER_FORCE_H2C=true I can successfully upload software installers > 32 MiB in size.