Signature & tests for fn load_from_file()

This function almost writes itself. I need a thin layer to handle the
file IO errors and report them appropriately, and then all the magic is
a pass-through of the existing read_conf_str.

I've made basic unit tests for the most obvious scenarios. The test for
missing-file behavior is incomplete because I need to create a new error
variant.

test_data/missing_all_table.toml

@@ -0,0 +1,5 @@
+# There must be an "[all]" table or the loader will reject the config file.
+
+["/some/other/path"]
+gitea_url = "fake-url"
+