Update IMessage.MergeFrom and MessageParser.ParseFrom to use zero-allocation spans for byte arrays

[jackhorton]

I was developing an application that caches protobuf messages serialized as byte arrays when I began looking into the various ways of deserializing those messages and noticed a discrepancy between the ReadOnlySpan<byte> implementation vs the byte[] implementation. This PR fixes that discrepancy by backing the byte[] and ByteString deserializers with the new zero-allocation ReadOnlySpan<byte> code path (introduced in #8473) rather than the CodedInputStream code path.

I wrote a small benchmark (not included in this PR but it can be if there is interest) to demonstrate the small perf win. Note that ByteArray still takes slightly longer than Span due to more aggressive inlining when using the span methods.

[MemoryDiagnoser]
public class MergeFromBytes
{
    public static readonly byte[] Serialized = new TestAllTypes
    {
        SingleBool = true,
        SingleBytes = ByteString.CopyFrom(1, 2, 3, 4),
        SingleDouble = 23.5,
        SingleFixed32 = 23,
        SingleFixed64 = 1234567890123,
        SingleFloat = 12.25f,
        SingleForeignEnum = ForeignEnum.ForeignBar,
        SingleForeignMessage = new ForeignMessage { C = 10 },
        SingleImportEnum = ImportEnum.ImportBaz,
        SingleImportMessage = new ImportMessage { D = 20 },
        SingleInt32 = 100,
        SingleInt64 = 3210987654321,
        SingleNestedEnum = TestAllTypes.Types.NestedEnum.Foo,
        SingleNestedMessage = new TestAllTypes.Types.NestedMessage { Bb = 35 },
        SinglePublicImportMessage = new PublicImportMessage { E = 54 },
        SingleSfixed32 = -123,
        SingleSfixed64 = -12345678901234,
        SingleSint32 = -456,
        SingleSint64 = -12345678901235,
        SingleString = "test",
        SingleUint32 = uint.MaxValue,
        SingleUint64 = ulong.MaxValue
    }.ToByteArray();

    [Benchmark]
    public TestAllTypes ByteArray() => TestAllTypes.Parser.ParseFrom(Serialized);

    [Benchmark]
    public TestAllTypes CodedInputStream() => TestAllTypes.Parser.ParseFrom(new CodedInputStream(Serialized));

    [Benchmark]
    public TestAllTypes Span() => TestAllTypes.Parser.ParseFrom(new ReadOnlySpan<byte>(Serialized));
}

BenchmarkDotNet=v0.13.2, OS=Windows 11 (10.0.22621.963)
AMD Ryzen 7 2700X, 1 CPU, 16 logical and 8 physical cores
.NET SDK=6.0.404
  [Host]     : .NET 6.0.12 (6.0.1222.56807), X64 RyuJIT AVX2
  DefaultJob : .NET 6.0.12 (6.0.1222.56807), X64 RyuJIT AVX2

Method	Mean	Error	StdDev	Gen0	Allocated
ByteArray	683.6 ns	1.59 ns	1.41 ns	0.3052	1.25 KB
CodedInputStream	748.8 ns	2.27 ns	2.01 ns	0.3462	1.41 KB
Span	663.7 ns	2.00 ns	1.87 ns	0.3052	1.25 KB

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