Making this change has a couple of benefits.
Firstly (and honestly the main one) is it make codegen much simpler. We
just to emit a base64 string and/or other embedded byte array to the
generated SDK. Currently we need emit a full `proto.Value` expression
for each language, and that's not hard but it's also not trivial and
means more combinations of things per test per language.
Secondly it will allow providers to use more efficient encodings of
their parameter than JSON if there is one. I imagine some providers
might make the parameter value a protobuf message and parse that
(similar to what we do for transform functions), but they can easily
fallback to just treating the bytes as a JSON string if they want.
The only downside to this is the value is obfuscated in the generated
SDK and in the state file. Neither of those are really expected to be
viewed by users, so this feels like a minor loss.
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