Refactor generated builder set methods to always take Option

[jdisanti]

This is a breaking change: All set_* methods on builders and clients will take an Option<T> where they sometimes used to take a T.

This is a refactor that will make it possible to simplify the code in #498. It also simplifies some existing code, such as the parsing of HTTP headers into non-optional values. Now the error handling for that lives in the builder rather than in the header parsing.

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[rcoh]

what if we added something like:

BuilderGenerator.callSetter(memberShape: MemberShape, symbol: Symbol) {
  rust("args")
}

that handles the option wrapping in one place? When we needed to force optionality, you could pass symbol.asOptional() (symbol.letIf(forceOptional) { it.asOptional() } is pretty clean)

As a separate issue, we probably should add builders for unions.

[jdisanti]

After examining it, I think I'm going to hold off on creating a common BuilderGenerator.callSetter. It would definitely benefit the SerdeJsonSerializerGenerator, but that's not going to be sticking around. I'll keep it in mind though as it may also benefit the new JsonSerializerGenerator in my other PR.