🔍 add scanner_signature_sources config option

This adds the option to configure which set of signatures `fdroid
scanner` should use, by configuring it in `config.yml`. It allows
fetching signatures in our custom json format. It also adds 3 additional
sources: 'suss', 'exodus', 'etip'

fdroidserver/common.py

@@ -163,6 +163,7 @@ default_config = {
     'archive_older': 0,
     'lint_licenses': fdroidserver.lint.APPROVED_LICENSES,  # type: ignore
     'git_mirror_size_limit': 10000000000,
+    'scanner_signature_sources': ['suss'],
 }