Enable string interpolation for hash-type parameters

[ChiliJohnson]

Is your feature request related to a problem? Please describe.

The FAQ mentions that you can use string interpolation to configure parameters using environment variables, e.g. some_field "#{ENV['FOO_HOME']}", but this doesn't seem to apply to keys or values in hash-type parameters.

Describe the solution you'd like

It would be nice if string values (or keys for that matter) in hash parameters could also get interpolated in the way string parameters already do. For example let's say I want to use an HTTP output with an authorization header that includes a token passed in through the environment:

<match>
  @type http
  headers {"Authorization": "Bearer #{ENV['SOME_ENV_VAR']}"}
</match>

Describe alternatives you've considered

For my particular example, a first-class feature to support bearer tokens in the HTTP output plugin (#3587) would likely solve my immediate problem, but also implementing more general-purpose string interpolation for hash parameters would remove the need for updates to that output plugin

Additional context

No response