Limit max age of Praefect connections to 15 minutes

By default, gRPC servers persist HTTP/2 connections indefinitely. However, load balancing doesn't work effectively if clients don't rotate connections between different Praefect servers. To prevent hotspots, limit the connection age to 15 minutes by default.

This commit introduces two configuration parameters for Praefect:

• max_connection_age (default: 15 minutes)
• max_connection_age_grace (default: infinity)

Relates to https://gitlab.com/gitlab-com/request-for-help/-/issues/2383