Add support for GitHub enterprise

[hculea]

Platform or tool

GitHub

Desired or expected behavior

The GitHub plugin should ensure that the GitHub CLI can talk to a custom host, when the user uses GitHub Enterprise.

Possible solutions:

1. When used with Github Enterprise, the GitHub CLI expects a GH_ENTERPRISE_TOKEN envvar rather than GH_TOKEN. User should be able to opt for overriding the default envvar by the enterprise-specific one.
2. Based on the GitHub host, shell plugins could determine whether GitHub enterprise is used and to inject the GH_ENTERPRISE_TOKEN envvar instead without any extra config required from the user.

Current behavior

GH_TOKEN always gets injected, regardless of what GH_HOST is.

Relevant log output

No response

op CLI version

No response