Add --output-file flag to get cluster action (#457)

Add --output-file flag to get cluster action to allow writing output
directly to a file. This makes it easier to invoke from a docker
container and will simplify many of the github actions.

Example usage:
```
$ flux-local get cluster --path tests/testdata/cluster -o yaml --output-file=/tmp/yaml
```

Tested manually, and implicitly through default argument using
`/dev/stdout`.
Fixes #455

flux_local/tool/format.py

@@ -2,8 +2,11 @@
 
 from typing import Generator, Any
 
+import sys
+from typing import TextIO
 import yaml
 
+
 PADDING = 4
 
 
@@ -47,10 +50,10 @@ def format(self, data: list[dict[str, Any]]) -> Generator[str, None, None]:
         for result in format_columns(cols, rows):
             yield result
 
-    def print(self, data: list[dict[str, Any]]) -> None:
+    def print(self, data: list[dict[str, Any]], file: TextIO = sys.stdout) -> None:
         """Output the data objects."""
         for result in self.format(data):
-            print(result)
+            print(result, file=file)
 
 
 class YamlFormatter:
@@ -63,6 +66,8 @@ def format(self, data: list[dict[str, Any]]) -> Generator[str, None, None]:
         ):
             yield line
 
-    def print(self, data: list[dict[str, Any]]) -> None:
+    def print(self, data: list[dict[str, Any]], file: TextIO = sys.stdout) -> None:
         """Format the data objects."""
-        print(yaml.dump_all(data, sort_keys=False, explicit_start=True), end="")
+        print(
+            yaml.dump_all(data, sort_keys=False, explicit_start=True), end="", file=file
+        )

flux_local/tool/get.py

@@ -171,12 +171,19 @@ def register(
             default="diff",
             help="Output format of the command",
         )
+        args.add_argument(
+            "--output-file",
+            type=str,
+            default="/dev/stdout",
+            help="Output file for the results of the command",
+        )
         args.set_defaults(cls=cls)
         return args
 
     async def run(  # type: ignore[no-untyped-def]
         self,
         output: str,
+        output_file: str,
         enable_images: bool,
         **kwargs,  # pylint: disable=unused-argument
     ) -> None:
@@ -216,7 +223,8 @@ async def run(  # type: ignore[no-untyped-def]
                     )
                     helm_content.update_manifest(manifest)
 
-            YamlFormatter().print([manifest.compact_dict()])
+            with open(output_file, "w") as file:
+                YamlFormatter().print([manifest.compact_dict()], file=file)
             return
 
         cols = ["path", "kustomizations"]
@@ -226,11 +234,15 @@ async def run(  # type: ignore[no-untyped-def]
             value["kustomizations"] = len(cluster.kustomizations)
             results.append(value)
 
-        if not results:
-            print(selector.not_found("flux cluster Kustmization", query.cluster))
-            return
+        with open(output_file, "w") as file:
+            if not results:
+                print(
+                    selector.not_found("flux cluster Kustmization", query.cluster),
+                    file=file,
+                )
+                return
 
-        PrintFormatter(cols).print(results)
+            PrintFormatter(cols).print(results, file=file)
 
 
 class GetAction: