[Feature] Support loading secrets from cloud providers (GCP, AWS, etc.)

[moosebay]

Checklist

• I have searched the repository issues and have not found a suitable solution or answer.
• I agree to the terms within the Contributor Covenant Code of Conduct.

Describe the problem you'd like to have solved

Allow users to load secrets directly from cloud secret managers using a syntax similar to our existing env var support.
Current behavior:

We support {{#env:client_secret}} to read from local environment variables

Describe the ideal solution

Proposed behavior:

GCP Secret Manager: {{#gcp:projects/p/secrets/oauth/versions/latest#client_secret}}
AWS Secrets Manager: {{#aws:secret-name#client_secret}} (future)
Azure Key Vault: {{#azure:vault/secret-name#client_secret}} (future)

Alternatives and current workarounds

No response

Additional context

No response