Support GitHub Enterprise Server hosted in a private environment

### Current Behavior

GitHub Advisory Database is mirrored from the GraphQL endpoint `https://api.github.com/graphql` using PAT linked to github.com.

### Proposed Behavior

GitHub Advisory Database is mirrored from a configurable GraphQL endpoint.
The GraphQL endpoint is modifiable on the GitHub Advisory configuration page, the value is typed in a text field, with a default value set to `https://api.github.com/graphql`.

Example of custom URL:
* `https://github.contoso.com/api/graphql`

### Checklist

- [X] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/master/CONTRIBUTING.md#filing-issues)
- [X] I have checked the [existing issues](https://github.com/DependencyTrack/dependency-track/issues) for whether this enhancement was already requested