[CLI] [request]: Support `heroku login` with manual credential entry in environments that do not have a browser

### Required Terms

- [X] I agree to follow this project's [Code of Conduct](https://github.com/heroku/roadmap/blob/main/CODE_OF_CONDUCT.md)
- [X] I have read and accept the [Salesforce Program Agreement](https://www.salesforce.com/company/program-agreement/)

### What service(s) is this request for?

Heroku CLI

### Tell us about what you're trying to solve. What challenges are you facing?

```markdown
I have a need to run the Heroku CLI in an environment where the browser is not accessible. However, the current `heroku login` command requires a browser if MFA/2FA is enabled.

My proposal is for the `heroku login -i` command, to prompt MFA/2FA users for the MFA token on the command line.
```