This check API only cheks the safekeeper_connstrings at the moment, and
the validation is limited to checking we have at least one entry in
there, and no duplicates.
## Problem
If the compute_ctl service is started with an empty list of safekeepers,
then hard-to-debug errors may happen at runtime, where it would be much
easier to catch them early.
## Summary of changes
Add an entry point in the compute_ctl API to validate the configuration
for safekeeper_connstrings.
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Co-authored-by: Heikki Linnakangas <heikki@neon.tech>
