CSHARP-5670: Implement ReadOnlyMemoryBsonReader

[BorisDog]

This PR introduces ReadOnlyMemoryBsonReader, an alternative to BinaryBsonReader. ReadOnlyMemoryBsonReader provides a more lightweight BSON deserialization from a continuous single chunk buffers.
Preliminary testing shows an improvement of ~50% in our deserialization benchmarks.

[Copilot]

Pull request overview

This PR introduces ReadOnlyMemoryBsonReader, a new, optimized BSON reader implementation designed for continuous single-chunk buffers. The reader provides approximately 50% improvement in deserialization benchmarks compared to BinaryBsonReader.

Key changes:

• New ReadOnlyMemoryBsonReader class that directly works with ReadOnlyMemory<byte> instead of streams
• New ReadOnlyMemoryBuffer implementation of IByteBuffer for read-only memory
• Updated existing BSON deserialization code to use BsonBinaryReaderUtils.CreateBinaryReader() which automatically selects the optimized reader when possible
• Enhanced name decoders (Utf8NameDecoder, TrieNameDecoder) to support span-based decoding
• Various refactorings to reduce nesting and improve code readability

Reviewed changes

Copilot reviewed 30 out of 30 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
ReadOnlyMemoryBsonReader.cs	New optimized BSON reader implementation for contiguous memory
ReadOnlyMemoryBuffer.cs	New read-only byte buffer wrapper for ReadOnlyMemory<byte>
ReadOnlyMemoryReaderSettings.cs	Settings class for the new reader
BsonBinaryReaderUtils.cs	Factory utilities for creating appropriate reader implementations
Utf8Helper.cs, Utf8NameDecoder.cs, TrieNameDecoder.cs	Span-based decoding support
RawBsonDocument.cs, RawBsonArray.cs, LazyBsonDocument.cs, LazyBsonArray.cs	Refactored to use factory method for reader creation
ObjectId.cs	Refactored to use ReadOnlySpan<byte> for parsing
BsonBinaryReader.cs	Moved common utilities to BsonBinaryReaderUtils
Various operation files	Simplified using statements and refactored to use factory method

Comments suppressed due to low confidence (1)

src/MongoDB.Bson/IO/ReadOnlyMemoryBsonReader.cs:1

• Double forward slashes in XML documentation comment. Should be /// <param name="requiredBsonType">The required BSON type.</param> with three slashes, not four.

/* Copyright 2010-present MongoDB Inc.

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[codeowners-service-app]

Assigned mongoKart for team dbx-csharp-dotnet because rstam is out of office.
Assigned jordan-smith721 for team dbx-csharp-dotnet because papafe is out of office.