main: version flag and optional metrics listener

[sr]

Mostly because I am FAFO about federating Fly's prometheus.

Version flag:

$ webauthn-oidc-idp -version
webauthn-oidc-idp, version devel (branch: metrics, revision: 497741168f27b872d2df5e002e5ee1b2dc20f96d-modified)
  build user:       root
  build date:       2024-03-12T13:10:20+00:00
  go version:       go1.22.1
  platform:         linux/arm64
  tags:             unknown

Build info metric:

webauthn_oidc_idp_build_info{app="sr-idp-staging", branch="metrics", goarch="amd64", goos="linux", goversion="go1.22.1", host="9a41", instance="7811952b5e5638", job="federate", region="ams", revision="85fc428c8e30de1c78938ffd964769084ee209e4", tags="unknown", version="devel"}

[sr]

runtime/debug.BuildInfo is nice but also kinda annoying: golang/go#29228 golang/go#50603