CSHARP-5531: Optimize the conversion from an array index to UTF-8 byte arrays

[dear-gary]

1. Use Utf8Formatter.TryFormat API to avoid string allocations and write directly to a reused byte[]
2. Define a byte[] member in BsonBinaryWriter class and reuse it to avoid byte[] allocations

seeCSHARP-5531 for more details

[BorisDog]

Thanks @dear-gary for this PR.
We'll be evaluating such allocations improvements in the serialization area in the scope of CSHARP-5575 in the upcoming quarter.
Closing this PR, as we plan to address this as part of a larger effort in Q2.