Rename AllowHTTP flag to be more clear

[BenjaminPelletier]

I feel like this flag may benefit to be renamed since it is only used to disable URLs validation. I would propose to rename it as AllowHTTP or AllowHTTPBaseUrls to prevent confusion. Indeed, I got mislead thinking that this was linked to the listening protocol of the DSS server.

Originally posted by @barroco in https://github.com/interuss/dss/pull/784#discussion_r890229815