Feature request: Nested secrets.

[MikeFindsThings]

Secrets should be able to be nested inside of other secrets.

For example, when deploying to Azure Kuberentes Service in a Github Action credentials are required in JSON format. Currently Github Actions secrets does allow and properly passes JSON secrets to this Action. However the JSON must include the required "clientID", "clientSecret", "subscriptionId", and "tenantId" to be added as string literals when entering the secret, like so:

{
  "clientId": "xxxxxxxxxxxxx",
  "clientSecret": "xxxxxxxxxxxxxxxxx",
  "subscriptionId": "xxxxxxxxxxxxxxxx",
  "tenantId": "xxxxxxxxxxxx"
}

A more dynamic approach would be to allow nested secrets:

{
  "clientId": "${{ secrets.CLIENT_ID }}",
  "clientSecret": "${{ secrets.CLIENT_SECRET }}",
  "subscriptionId": "${{ secrets.SUBSCRIPTION_ID }}",
  "tenantId": "${{ secrets.TENANT_ID }}"
}

This allows for only having to update a single secret if any of the values change.