Improve SocketAddress.ToString() performance

[stephentoub]

Following up on request/feedback at #30521 (comment).

Microbenchmark:

using System;
using System.Net;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

[MemoryDiagnoser]
public class Program
{
    static void Main(string[] args) => BenchmarkSwitcher.FromTypes(new[] { typeof(Program) }).Run(args);

    private readonly SocketAddress _addr = new IPEndPoint(IPAddress.Parse("40.112.72.205"), 443).Serialize();

    [Benchmark]
    public string SAToString() => _addr.ToString(); 
}

Method	Mean	Error	StdDev	Median	Gen 0	Gen 1	Gen 2	Allocated
Before	528.7 ns	10.67 ns	24.73 ns	517.1 ns	0.1183	-	-	496 B
After	346.6 ns	4.733 ns	3.695 ns	0.0362	-	-	-	152 B