Support for Migrating from Bitbucket to GitHub Enterprise Server

## Description
Hello! We are in the process of migrating thousands of repositories from Bitbucket to GitHub Enterprise Cloud. However, due to legal and security reasons, some repositories need to be transferred to GitHub Enterprise Server.

Currently, in bbs2gh, the gh CLI only supports GitHub Enterprise Cloud and github.com as targets.
Is it possible to also include support for GitHub Enterprise Server?

It seems feasible to call the GraphQL API in GitHub Enterprise Server, but it would be really helpful if gh-gei could support this.

Ideal solution
Add support for a --ghes-api-url option in the gh bbs2gh migrate-repo operation.
This might be related to the following code: https://github.com/github/gh-gei/blob/main/src/bbs2gh/Commands/MigrateRepo/MigrateRepoCommand.cs#L215