Added possibility to trigger workflow manually

Each workflow should be malually triggerable and should trigger its
downstream dependencies. This commit adresses the first part of that
statement. Triggering of downstream dependencies is not yet developed,
as downstream workflows aren't yet created.

.github/workflows/client.yml

@@ -6,6 +6,7 @@ on:
     branches:
       - master
   pull_request:
+  workflow_dispatch:
 
 jobs:
   build-and-test: