Use env vars without it needing to be the entire string.

[icopp]

Is your feature request related to a problem? Please describe.
When using branching, it's impossible to properly supply various values based on env vars but also other text.

For example:

[auth.external.google]
[auth]
enabled = true
site_url = "env(SUPABASE_AUTH_SITE_URL)"
additional_redirect_urls = ["env(SUPABASE_AUTH_SITE_URL)/auth/callback"] # This doesn't work!

Describe the solution you'd like
There should be some way to use env() placeholders inline in a string, either as-is or with extended syntax (for example, ${env()} like JS template strings).

Describe alternatives you've considered
Right now, the only alternative is to hardcode everything, which obviously doesn't work for branching.

Additional context
https://github.com/orgs/supabase/discussions/18937