Support private instances

[markatk]

Detailed Description

Add support for private instances of GitHub or GitLab.

Context

We use a self-hosted gitlab instance at work and want to use this tool. We already use GitVersion and this would match pretty good to our current CI toolchain.
This change would enable other users who use different gitlab or github enterprise instances to include GitReleaseManager in their toolchains.

Possible Implementation

Add an option to change the host url.

Your Environment

• Version Used: Not yet deployed since it does not work with private instances.
• Edition Used (.NET Core, .NET Framework): .NET 7/8
• Operating System and version (Windows 10, Ubuntu 18.04): Windows 10, Windows Server 2019/2022, Debian 10/11