Support for Self-Hosted Providers

[eliatcodecov]

Codecov has an on-premises offers that allows it to work with self hosted versions of GitHub, GitLab, and BitBucket. We should support this use case eventually. To support them, the extension will need to accept the following input parameters in its settings:

1. The URL of the self-hosted SCM install and its type (github, bitbucket, or gitlab)
2. The URL of the self-hosted Codecov API install, such that we have a service to make API calls against that isn’t the default api.codecov.io.

With these two items, self-hosted support should be straightforward and will leverage much of the same logic and functionality as the cloud supported extension.