Reduce DefaultSubmissionClient allocations

[benaadams]

• Don't allocate a response for the common paths e.g. SubmissionResponse == Ok 200
• Use interpolated strings in preference as that will have a non-boxing; low allocating path in C#10 API Proposal: C# 10 interpolated strings support, part 2 dotnet/runtime#50635

[ejsmith]

Nice! I really like the change of using interpolated strings. It's a much cleaner syntax so I'm glad they are going improve the perf. There used to be a lot of debate on when to do string concat vs stringbuilder vs string interpolation. It will be good to just say always use string interpolation.