Write raw results from flows

[desertaxle]

This PR updates flows to write serialized versions of their return values without persisting any metadata to disk. Previously, flows would write result files containing metadata like the serializer used to create the file's result payload. The same info already persisted to the API is used when calling .result on a flow's state.

To have the ResultStore persist only raw results, I created a NullFileSystem class that looks like a WritableFileSystem, but doesn't do anything. The ResultStore for each flow is given a NullFileSystem for metadata_storage, which results in no metadata being written.

To allow a ResultStore to handle raw results, a serializer kwarg is added to the read methods. This option allows the caller to manually provide which serializer to use to deserialize the result. The new kwarg is used internally when loading a result via a flow state's .result method.

[codspeed-hq]

CodSpeed Performance Report

Merging #15434 will not alter performance

_{Comparing raw-flow-results (6a1c262) with main (d82cc43)}

Summary

✅ 3 untouched benchmarks

[cicdw]

yea i like this approach - prevents any hard-coding that we can't walk back from, but achieves the desired result (ha) for now