Make geolocation selectable with a no-location default

[jwrosewell]

User story

As a deployer, I want to choose whether and how location is resolved, so that the default resolves no location and I opt into the host's geo explicitly.

Description

Select the geolocation source by configuration through the existing PlatformGeo interface. The default resolves no location, so the default path makes no host geo call. The host platform's own geo lookup is opt-in by configuration.

Done when

• The geo source is selected by [geo] provider.
• The default resolves no location and makes no host geo call.
• The host platform geo is opt-in.
• A selected-but-unknown provider is rejected at startup.

References

• Part of Pluggable, competition-neutral providers with a technical permission model #777. Implemented by the neutral-providers PR.
• Related: Clean up dead GDPR country code in geo.rs and reconcile Eid/Uid types #518 (dead GDPR country code in geo.rs) and Thread RuntimeServices into downstream handlers #598 (RuntimeServices/geo threading).