Backend changes for migrating "Import from URL" to vue

As part of the "New project creation" form/flow, we should migrate the "Import from URL" page to Vue. This will become available behind the new_project_creation_form.

This is related to #514700 (closed)

Backend changes

To support the git, HTTP, and HTTPS schemas, we can change the Import::ValidateRemoteGitEndpointService service to use Gitlab::GitalyClient::RemoteService.exists? method instead of using Gitlab::HTTP to discover if the repositories exist

Follow example on how to use the Gitlab::GitalyClient::RemoteService.exists? method

HTTPS - Public

Gitlab::GitalyClient::RemoteService.exists?("https://github.com/rails/rails")

HTTPS - Private

Gitlab::GitalyClient::RemoteService.exists?("https://rodrigotomonari:ACCESS_TOKEN@github.com/rodrigotomonari/test-private.git")

GIT - Public

Gitlab::GitalyClient::RemoteService.exists?("git://git.kernel.org/pub/scm/bluetooth/bluez.git")

The method only returns true or false. Therefore, we may not know if the credentials are invalid or if the URL is invalid. When the method returns false, we should return a generic message stating that it was not possible to access the repository.