Add signal handler and shutdown hook

[tpoliaw]

When the application is running in a container, it is given PID 1 which
means it does not automatically get given signal handlers and can't be
killed. When running the container in the foreground this stops Ctrl-C
from interrupting it while in kubernetes it means the graceful shutdown
always times out resulting in the container eventually being forcibly
deleted, killing the process.

Adding an explicit handler for the common signals used to interrupt
processes means that Ctrl-C, kubernetes and podman stop all work as
expected.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.08%. Comparing base (2800800) to head (6021dbf).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/graphql/mod.rs	0.00%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #232      +/-   ##
==========================================
- Coverage   77.52%   77.08%   -0.45%     
==========================================
  Files          13       13              
  Lines        1909     1920      +11     
==========================================
  Hits         1480     1480              
- Misses        429      440      +11     

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