Add extraVolumes and extraVolumeMounts config

[consideRatio]

This can be considered done, if the binderhub-service chart gets extraVolumes and extraVolumeMounts config added to influence its deployment pod and the container running binderhub.

This functionality is generally useful, and can for example be used to mount a GCP service account key to be consumed by google-cloud-logging library to submit logs about launch event.

Practical input

• doing this is very similar to doing the changes in Add extraEnv config for the deployment pod #96

• in Add extraEnv config for the deployment pod #96 the tpl function was used, it makes us able to provide configuration that is itself something the helm template can render, and that can be useful - so mimic its inclusion also for extraVolumes and extraVolumeMounts

• to test and develop changes, make use of commands below - the tools/ commands are available in
  z2jh / binderhub as well btw

  # generates a .json version of the values.schema.yaml which is respected by the helm CLI
  # (the YAML version is just to make work with the schema file, which must be .json, easier)
  tools/generate-json-schema.py
  
  # this script validates the chart's default values against the schema but also the
  # example config values in tools/templates/lint-and-validate-values.yaml
  tools/validate-against-schema.py
  
  # test render the chart locally, showing specifically the rendered deployment.yaml template
  # to modify developing extraVolumes and extraVolumeMounts
  helm template binderhub-service --show-only templates/deployment.yaml
  
  # use this command to render the chart, focusing on whats rendered after
  # `volumes:` string in the deployment resource, updating every second.
  # This is useful when modifying the template file to verify changes quickly
  watch -n1 'helm template binderhub-service --show-only templates/deployment.yaml | grep -A10 "volumes:"'

[yuvipanda]

This has become much lower priority due to 2i2c-org/infrastructure#4343. I think I'm mostly ok with this not being done as well, as it no longer necessarily serves any product function.