Feature: add "Pull mirror API"

[mikaelol]

The pull mirror API is missing, only remote mirror is currently implemented (which is the push mirror).

[jmini]

Project remote mirrors API is:

gitlab4j-api/gitlab4j-api/src/main/java/org/gitlab4j/api/ProjectApi.java

Lines 4382 to 4428 in 8259f86

	/**
	* Get all remote mirrors and their statuses for the specified project.
	*
	* <pre><code>GitLab Endpoint: GET /projects/:id/remote_mirrors</code></pre>
	*
	* @param projectIdOrPath the project in the form of an Long(ID), String(path), or Project instance
	* @return a list of project's remote mirrors
	* @throws GitLabApiException if any exception occurs
	*/
	public List<RemoteMirror> getRemoteMirrors(final Object projectIdOrPath) throws GitLabApiException {
	return (getRemoteMirrors(projectIdOrPath, getDefaultPerPage()).all());
	}
	/**
	* Get a Pager of remote mirrors and their statuses for the specified project.
	*
	* <pre><code>GitLab Endpoint: GET /projects/:id/remote_mirrors</code></pre>
	*
	* @param projectIdOrPath the project in the form of an Long(ID), String(path), or Project instance
	* @param itemsPerPage the number of items per page
	* @return a Pager of project's remote mirrors
	* @throws GitLabApiException if any exception occurs
	*/
	public Pager<RemoteMirror> getRemoteMirrors(final Object projectIdOrPath, int itemsPerPage)
	throws GitLabApiException {
	return (new Pager<RemoteMirror>(
	this,
	RemoteMirror.class,
	itemsPerPage,
	null,
	"projects",
	getProjectIdOrPath(projectIdOrPath),
	"remote_mirrors"));
	}
	/**
	* Get a Stream of all remote mirrors and their statuses for the specified project.
	*
	* <pre><code>GitLab Endpoint: GET /projects/:id/remote_mirrors</code></pre>
	*
	* @param projectIdOrPath the project in the form of an Long(ID), String(path), or Project instance
	* @return a Stream of project's remote mirrors
	* @throws GitLabApiException if any exception occurs
	*/
	public Stream<RemoteMirror> getRemoteMirrorsStream(final Object projectIdOrPath) throws GitLabApiException {
	return (getRemoteMirrors(projectIdOrPath, getDefaultPerPage()).stream());
	}

Indeed the pull mirror API https://docs.gitlab.com/ee/api/project_pull_mirroring.html can be contributed to this project