Improve perf of base64 encoding check on a consumer hot path

[Havret]

This PR improves the performance of the Base64 encoding check, which is performed for every message processed by the consumer. This change complements the work done in #2370.

Below are the results of benchmarking the new implementation (Base64.IsValid) against the existing Convert.FromBase64String approach:

| Method           | Mean          | Error        | StdDev       |
|----------------- |--------------:|-------------:|-------------:|
| IsBase64         |      24.52 ns |     0.074 ns |     0.066 ns |
| FromBase64String | 138,883.49 ns | 2,535.710 ns | 2,247.841 ns |

The code used to produce the benchmark results::

class Program
{
    static void Main(string[] args)
    {
        BenchmarkRunner.Run<IsBase64StringBenchmark>();
    }
}

public class IsBase64StringBenchmark
{
    private readonly string _invalidBase64 = string.Join("", Enumerable.Repeat("This is not base64!", 10000));

    [Benchmark]
    public bool IsBase64()
    {
        return Base64.IsValid(_invalidBase64);
    }

    [Benchmark]
    public bool FromBase64String()
    {
        try
        {
            _ = Convert.FromBase64String(_invalidBase64);
            return true;
        }
        catch (FormatException)
        {
            return false;
        }
    }
}

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