[Proposal] add search interface for go-scm

[colynn]

we know go-scm can work well with Drone, and it can use repo.Find interface to find a specific project.

In our scenario, we hope can use string to search the project list, so whether we can add a search interface for go-scm,

interface

# https://github.com/go-atomci/go-scm/blob/master/scm/search.go#L16
	SearchService interface {
		// Find returns a repository by name.
		FindProjects(context.Context, string) ([]*Repository, *Response, error)
	}

gitlab sample

# gitlab
# https://github.com/go-atomci/go-scm/blob/master/scm/driver/gitlab/search.go#L18
func (s *searchService) FindProjects(ctx context.Context, projectName string) ([]*scm.Repository, *scm.Response, error) {
	path := fmt.Sprintf("api/v4/search?scope=projects&search=%s", encode(projectName))
	out := []*repository{}
	res, err := s.client.do(ctx, "GET", path, nil, &out)
	return convertRepositoryList(out), res, err
}