Support configuring fetch-artifacts-url.yaml

This allows user to define `fetch-artifacts-url.yaml` in ocp-build-data image config:

```yaml
content:
  source:
    artifacts:
      from_urls:
      - target: yarn-v1.22.19.tar.gz
        url: http://download.eng.bos.redhat.com/rcm-guest/puddles/RHAOS/artifacts/build-deps/yarn-v1.22.19.tar.gz
        sha256: 732620bac8b1690d507274f025f3c6cfdc3627a84d9642e38a07452cc00e0f2e
        source-url: http://download.eng.bos.redhat.com/rcm-guest/puddles/RHAOS/artifacts/build-deps/yarn-source-v1.22.19.tar.gz
        source-sha256: 50af0025d2ef942bf3e6cdd0587dc9c4dd8d6e6e69501b84935d09f2b2b5de8b
```

doozerlib/distgit.py

@@ -418,7 +418,7 @@ def __init__(self, metadata, autoclone=True,
         self.build_lock.acquire()
         self.rebase_event = Event()
         self.rebase_status = False
-        self.logger = metadata.logger
+        self.logger: logging.Logger = metadata.logger
         self.source_modifier_factory = source_modifier_factory
 
     def clone(self, distgits_root_dir, distgit_branch):
@@ -442,6 +442,24 @@ def image_build_method(self):
 
         return build_method
 
+    def _write_fetch_artifacts(self):
+        # Write fetch-artifacts-url.yaml for OSBS to fetch external artifacts
+        # See https://osbs.readthedocs.io/en/osbs_ocp3/users.html#using-artifacts-from-koji-or-project-newcastle-aka-pnc
+        config_value = None
+        if self.config.content.source.artifacts.from_urls is not Missing:
+            config_value = self.config.content.source.artifacts.from_urls.primitive()
+        path = self.dg_path.joinpath('fetch-artifacts-url.yaml')
+        if path.exists():  # upstream provides its own fetch-artifacts-url.yaml
+            if not config_value:
+                self.logger.info("Use fetch-artifacts-url.yaml provided by upstream.")
+                return
+            raise ValueError("Image config option content.source.artifacts.from_urls cannot be used if upstream source has fetch-artifacts-url.yaml")
+        if not config_value:
+            return  # fetch-artifacts-url.yaml is not needed.
+        self.logger.info('Generating fetch-artifacts-url.yaml')
+        with path.open("w") as f:
+            yaml.safe_dump(config_value, f)
+
     def _write_osbs_image_config(self, version: str):
         # Writes OSBS image config (container.yaml).
         # For more info about the format, see https://osbs.readthedocs.io/en/latest/users.html#image-configuration.
@@ -1499,6 +1517,8 @@ def update_distgit_dir(self, version, release, prev_release=None, force_yum_upda
 
             self._write_cvp_owners()
 
+            self._write_fetch_artifacts()
+
             dfp = DockerfileParser(path=str(dg_path.joinpath('Dockerfile')))
 
             self._clean_repos(dfp)