feat: integrate tool with gitlab service (#39)

* feat: integrate tool with gitlab service

Fix https://github.com/lampajr/backporting/issues/30

src/service/git/git-client.ts

@@ -0,0 +1,35 @@
+import { BackportPullRequest, GitPullRequest } from "@bp/service/git/git.types";
+
+/**
+ * Git management service interface, which provides a common API for interacting 
+ * with several git management services like GitHub, Gitlab or Bitbucket.
+ */
+ export default interface GitClient {
+
+  // READ
+
+  /**
+   * Get a pull request object from the underneath git service
+   * @param owner repository's owner
+   * @param repo repository's name
+   * @param prNumber pull request number
+   * @returns {Promise<PullRequest>}
+   */
+  getPullRequest(owner: string, repo: string, prNumber: number): Promise<GitPullRequest>;
+
+  /**
+   * Get a pull request object from the underneath git service
+   * @param prUrl pull request html url
+   * @returns {Promise<PullRequest>}
+   */
+   getPullRequestFromUrl(prUrl: string): Promise<GitPullRequest>;
+
+  // WRITE
+
+  /**
+   * Create a new pull request on the underneath git service
+   * @param backport backport pull request data
+   * @returns {Promise<string>} the pull request url
+   */
+  createPullRequest(backport: BackportPullRequest): Promise<string>;
+}