Add ability to generate secrets for the application

[lorenyu]

Ticket

Resolves #562

Changes

⚠️ breaking change (change to way secrets are defined in app-config in environment-variables.tf).
secrets are now defined as a map:

secrets = {
  ENV_VAR_NAME = {
    manage_method     = "code" or "manual"
    secret_store_path = "/ssm/param/name"
  }
}

It was previously defined as a list:

secrets = [
  {
    name = "ENV_VAR_NAME",
    ssm_param_name = "/ssm/param/name"
  }
]

• Add new module modules/secret for generating new secrets or referencing existing secrets
• Refactor interface

To migrate:

• In app-config's environment-variables.tf, update secret definitions to use the new format.
• For secrets managed outside of the project's codebase, set manage_method = "manual"
• For secrets created within the project's codebase but defined elsewhere, move (using terraform mv) the aws_ssm_parameter to module.secret[ENV_VAR_NAME].aws_ssm_parameter.secret

Context for reviewers

This is the work for #562

The CI failures for trivy and anchor aren't related to this PR.

Testing

Developed and tested in platform-test in this PR: navapbc/platform-test#95

Rollout notes

After the template-infra PR is merged, we should update the platform-test PR from main and then merge it rather than closing it, since it'll have changes to app-config and the app that wouldn't have been pulled over from the template CD.

Future work

In the future we should add the ability to rotate the randomly generated secret by adding version numbers: #601

[rocketnova]

Thank you for working on this! Left some feedback on naming, only one of which isn't a nit.