feat: Add support for GitHub Connections in steps and step templates

[liamhughes]

Currently it is not possible to use the client to create or modify deployment processes that use steps that authenticate to a Git repository via a GitHub connection. Nor is it possible to create or modify step templates using a GitHub connection.

In both cases one will receive the following error: Octopus API error: There was a problem with your request. [GitHub connection must be specified when using a GitHub repository source.].

This PR adds support by adding the GitHubConnectionId field to GitDependency.

Example usage:

action.GitDependencies = []*gitdependencies.GitDependency{
    {
        Name:               "",
        RepositoryUri:      "https://github.com/example/octopus-github-web-app.git",
        DefaultBranch:      "main",
        GitCredentialType:  "GitHub",
        GitHubConnectionId: "GitHubAppConnections-1",
    },
}