Support the Accept-Conjure-Error-Parameter-Format header

[sfackler]

Before this PR

Only the server side (at codegen time) could pick if error parameters should use the old stringification approach or not.

After this PR

==COMMIT_MSG==
Added support for clients to opt-in to the new error parameter serialization format with the Accept-Conjure-Error-Parameter-Format header.
==COMMIT_MSG==

This should mirror the logic of the Java side: palantir/conjure-java#2588

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Description

Added support for clients to opt-in to the new error parameter serialization format with the Accept-Conjure-Error-Parameter-Format header.

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[sfackler]

Had to do some refactoring here since we don't have access to request headers after the handler runs.

[sfackler]

Gonna hold off on this for a little until we're more confident that we want to commit to this on the Java side.

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