Clients provide version to SpacetimeDB via header

[cloutiertyler]

John: SpacetimeDB should support reading from a new header, X-Client-Version, which contains the client’s supported SDK version. Important: this version should match the SDK’s supported SpacetimeDB version and may not exactly match the actual SDK version. For example, SDK version 0.5.1, 0.5.1.1 or 0.5.2 might report they support SpacetimeDB version 0.5.1.

Tyler: X-Client-Version <- this is the old way of doing things with custom header names, please check the relevant RFC