Env file customization from aspire cli commands

### Is there an existing issue for this?

- [ ] I have searched the existing issues

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

I am looking for the support where I can generate the pre-loaded env file with my custom env_var values.

### Describe the solution you'd like

I am using aspire in my new production grade application. Where now I am researching and setting up the deployment flow.
I have used some external dependencies which have credentials, to provide these sensitive and secret information, aspire is supporting the environment variable.
Through parameter with secret: true, we can add the parameters and reference it with the env variable. And aspire will know that at publish time these values will be resolved by the env file.

Now when I do aspire publish, it is generating an empty file.
with aspire do prepare-env, it is generating the env file with random values.

I want to set up the deployment pipeline and for the same, I am looking for something through which we can pass the secrets and it will replace the random values of env with given values. So I will just have to replace the new env file with existing one in the production instance.

Let me know by anything we can achieve this goal.

### Additional context

_No response_