Draft: Add ability to specify custom yaml values to the Big Bang Helm Chart

What/Why

• Add ability to specify custom yaml values by adding them to the terraform secret in the bigbang namespace. We were already using that secret to set the registry credentials. Now we can also specify custom values for the chart.
• Add descriptions to the inputs and outputs
• Minor renaming of files for developer quality of life and maintainability

Open questions

1. Previous development here seems to prefer passing paths to files. We could potentially modify the new var.custom_helm_values to accept the path to a file instead of a raw yaml string.
2. Added a TODO to resource kubernetes_secret.bb-common-secret that it should be renamed. We can (a) leave the TODO for now, (b) rename it, or (c) leave it alone and take out the TODO.
3. Added a bunch of "this works, but..." for the new variable. Needs more thought.

DON'T put 'registryCredentials' here. That's what the 'registry_credentials' variable is for. NOTE: The 'terraform' secret is the first to be applied, so any keys that appear in this will be overwritten if they also appear in a later values file that is used. Also, these custom values will not be tracked by GitOps, so you'll have to do another 'terraform apply' if you change them.

Wishlist for future MRs

1. Add a pre-commit config that automatically does terraform fmt, tflint, tfsec, and terraform-docs (#2 (closed))
2. Add a CI pipeline (#3 (closed))

Closes #1 (closed)