[C#] GetByKey needs an allocation-free overload

[CTVjweiss]

I'm using Flatbuffers in C# because I want an allocation free way of accessing my data. However, the current implementation of GetByKey() has only one pathway that takes in a string and converts it to a newly allocated byte[] in the generated __lookup_by_key function.

In my opinion this goes against the fundamental design principles of FlatBuffers of minimizing allocations whenever possible. I think it would be pretty easily fixed doing one of the following:

1. Allow the caller to pass in an ArraySegment or a byte[] with offset and length that already points to the UTF8 encoded string.
2. Add a byte[] parameter that defaults to null for GetByKey() that will be used for encoding to UTF8 for the binary search. If passed in, rely on the caller having passed a large enough byte array to the conversion to use the UTF8.GetBytes() function that writes to an existing array.

[dbaileychess]

Feel free to submit a PR for this.

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