Unify some of the configuration for the client and server tools

[martinthomson]

As I noted in #1402, there is a lot of common stuff between the client and server command-line tools. It would be good if we could unify some of this.

It might make sense to move this to a module under the mainline code (both neqo-transport and neqo-http3), covered by feature flags that the command-line tools enable. Then, a standard build won't pull in a bunch of dependencies, but we have a single place for this stuff.

[larseggert]

CC @mxinden, since you're touching that code now.

[mxinden]

Close here in favor of the recent #1696?

(Sorry for creating a ~duplicate in the first place.)

[larseggert]

Sure.