Add support for running an embedded cloud_sql_proxy

[sritchie]

This PR:

• adds a script to resources that is capable of running a cloud_sql_proxy instance inside the container
• adds support for injecting this script into the generated Dockerfile

This does this always, though we may want to control the behavior with a flag. @ajslone , I moved the resources into their own folder inside the container so we don't mount over them when we run caliban shell. This way if a user wants they can test the behavior with a command inside caliban shell.

[codecov]

Codecov Report

Merging #60 into master will decrease coverage by 0.17%.
The diff coverage is 44.89%.

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
- Coverage   51.58%   51.41%   -0.18%     
==========================================
  Files          28       30       +2     
  Lines        2929     2970      +41     
==========================================
+ Hits         1511     1527      +16     
- Misses       1418     1443      +25     

Impacted Files	Coverage Δ
caliban/docker/build.py	33.50% <21.05%> (-0.37%)	⬇️
caliban/resources/cloud_sql_proxy.py	52.00% <52.00%> (ø)
caliban/util/metrics.py	100.00% <100.00%> (ø)
caliban/platform/gke/util.py	70.38% <0.00%> (-1.93%)	⬇️
caliban/util/fs.py	89.62% <0.00%> (-0.95%)	⬇️
caliban/platform/cloud/types.py	89.47% <0.00%> (-0.66%)	⬇️
caliban/util/auth.py	76.19% <0.00%> (+9.52%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5c9efd5...6f3d41c. Read the comment docs.

[ajslone]

LGTM