Add C# codegen for Clear message to reuse message objects in a pool

[batesj]

What language does this apply to?
C# codegen.

Describe the problem you are trying to solve.
Using protobufs in Unity can cause excessive garbage collection overhead because of all the new class instances created while parsing a proto into a message instance.

Describe the solution you'd like
I would like to be able to reuse a pool of message instances repeatedly for parsing protos.

Describe alternatives you've considered
I could not find examples, but perhaps there is a way to do it with reflection? Of course, if that's expensive then it is replacing garbage with CPU overhead.

[ifshuaishuai]

In game developing area, especially in unity, recycle message object from pool is a must-have. I'm current looking for same Clear() method like PB did for Java for recycling.

[danielhedren]

This is a much-wanted feature in the context of real time bidding as well since we're normally parsing and creating thousands of messages per second.

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