Add a util package for length-delimited proto streams

[bduffany]

• Add a package protodelim for consuming length-delimited proto streams. The name is inspired by the discussion here and also follows a similar naming convention to proto packages that deal with proto encodings (protojson, protowire, etc.)
• Use the package in cli/printlog and server/util/protofile. Will also be used in Allow workflows to attach invocation artifacts #3386
  This should also reduce the amount of allocations in ReadProto (in protofile.go) since the new util reuses the same buffer across all unmarshal operations.

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Version bump: Patch

[bduffany]

Closing in favor of google.golang.org/protobuf/encoding/protodelim