PipeWriter.Advance with prefix discard

[scalablecory]

This API would allow PipeWriter to be used efficiently with protocols which use variable-length enveloping. Without this, one must first buffer data elsewhere before writing into a PipeWriter.

The basic idea is to write code that over-allocates a prefix for the envelope:
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And then writes the data:
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And finally writes the variable-length envelope into the prefix:
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And then calls Advance() with the number of unused prefix bytes.

Or:

const int maximumEnvelopeLength = 4;

PipeWriter writer = ...;
Span<byte> span = writer.GetSpan(...);

int dataLength = WriteData(span.Slice(maximumEnvelopeLength));
int actualEnvelopeLength = WriteDataEnvelope(dataLength, span.Slice(0, maximumEnvelopeLength));

writer.Advance(dataLength + actualEnvelopeLength, discard: maximumEnvelopeLength - actualEnvelopeLength);

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