Title:

Add a flag to disable extensions at startup.

Description:

Many (all?) extensions can be removed from Envoy, but typical Docker images distributed by the Envoy project and getenvoy.io target a wide audience and can't be aggressive in removing extensions.

My proposal is to add a command-line flag --disable-extensions=NAME[,NAME...] to envoy so that the named extensions can be disabled (by nulling the pointer in the relevant factory registry) early in startup, before any configuration is parsed.

If the named extension isn't present in the envoy build, the startup check will log at warning level (i.e. it is not a fatal error).