Refit should emit diagnostics for invalid interfaces

[TimothyMakkison]

Refit should emit diagnostic warnings and errors for invalid methods and interfaces. Many exceptions in RequestBuilderImplementation could be statically determined via a source generator.

• Invalid url backslash
• Multiple CancellationToken params
• HeadersCollection must be IDictionary

Severity to be determined layer.

More advanced checks may to read the attribute values. It isn't required but I'd prefer to reference ../Refit.csproj and construct each attribute in the source generator. I don't know if this could cause issues 🤔

Would this be considered to be a breaking change? It may prevent users from building code that previously ran, although it was likely invalid.