Add a UTF-8 statistics handler

[verdie-g]

The statistics payload can be quite large (several MB) and currently it can be copied twice in the managed code:

1. The payload is received from the C driver
2. A .NET string is allocated for the JSON
3. The statistics handler is called
4. The subscriber will likely call System.Text.Json.Serialize.Deserialize which cause the string to be converted back to UTF-8

This round-trip causes important memory fragmentation which puts pressure on the GC. In .NET 6, this can be avoided using Span.

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