Add support to Github Enterprise

[malealva]

Hello everyone,

I encountered some difficulties using the pysnyk library with GitHub Enterprise, so I ended up making this code change that made it work (at least for what I needed). I believe it might be helpful.
I couldn't find any open issues on the topic. Feel free to close the PR if it's not relevant.
Also, I may have missed something, so any suggestions for corrections or improvements will be appreciated.

Thank you.

About the change:

The Snyk API calls to import projects from "github.com" and "api.github.com" seem to be the same. I was able to make it work with the following snippet:

def import_github_repo(snyk_org_id, github_org_name, repo_name, branch):
    org = get_snyk_client().organizations.get(snyk_org_id)
    integration = org.integrations.filter(name="github-enterprise")[0]
    return integration.import_git(github_org_name, repo_name, branch)

To automate this and make it work with import_project, I added support for handling "api.github.com" and detecting the integration associated with github-enterprise when necessary. This will make it possible to capture the correct Integration ID and make the call to import_git seamlessly.