Implement proc-macro-api versioning
So as it stands, we can't really change the proc-macro-api protocol at all without breaking all proc-macro servers again. To somewhat alleviate this we can move the supported ABI mess over to the proc-macro-api now by supporting multiple versions there (versions defined by us at least, not by rustc). Since the proc-macro-api protocol has no versioning scheme at the moment though, the best we can do here is add a new request to query the version from a server. Due to how the server currently works though, if it encounters an unknown request it will exit, meaning we can check if it is a server without support by checking if it exited after our version check request, that way we can support the current circulating server as well.
We need this since our span type will change from `TokenId` to something else at some point, but for that to work we need to comply with that the server expects. So knowing the version the server is using we can decide whether to send our new span data, or the tokenid (assuming we keep that information with our span data as well, alternatively we send irrelevant tokenids). That way we can keep old servers working while the user installations slowly migrate to newer servers that support the new spandata.
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