Support External SASL configuration

[Gsantomaggio]

Support External SALS Auth

How to test:

var ssl = new SslOption()
        {
            Enabled = true,
            ServerName = "my_server_name",
            CertPath = "certs/client/keycert.p12",
            CertPassphrase = null,
            CertificateValidationCallback = (sender, certificate, chain, errors) => true,
        };

        var config = new StreamSystemConfig()
        {
            UserName = "user_does_not_exist",
            Password = "password_does_not_exist",
            Ssl = ssl,
            Endpoints = new List<EndPoint>(new List<EndPoint>()
            {
                new DnsEndPoint("my_server_name", 5551)
            }),

            AuthMechanism = AuthMechanism.External,
        };
        const string stream = "test-stream";
        var system = await StreamSystem.Create(config);
        await system.CreateStream(new StreamSpec(stream));
        Console.WriteLine("Stream created");

Docker image:

pivotalrabbitmq/rabbitmq:v3.12.x-otp-max-bazel

or server version for this fix rabbitmq/rabbitmq-server#8488

[codecov]

Codecov Report

Patch coverage: 90.00% and project coverage change: -0.15 ⚠️

Comparison is base (a209be3) 93.09% compared to head (437d6b5) 92.94%.

❗ Current head 437d6b5 differs from pull request most recent head 2746a9f. Consider uploading reports for the commit 2746a9f to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #274      +/-   ##
==========================================
- Coverage   93.09%   92.94%   -0.15%     
==========================================
  Files         104      104              
  Lines        9120     9139      +19     
  Branches      720      722       +2     
==========================================
+ Hits         8490     8494       +4     
- Misses        484      497      +13     
- Partials      146      148       +2     

Impacted Files	Coverage Δ
RabbitMQ.Stream.Client/StreamSystem.cs	83.07% <70.00%> (-0.13%)	⬇️
RabbitMQ.Stream.Client/Client.cs	92.18% <100.00%> (+0.14%)	⬆️
RabbitMQ.Stream.Client/ClientExceptions.cs	70.73% <100.00%> (+2.31%)	⬆️
RabbitMQ.Stream.Client/RawConsumer.cs	85.67% <100.00%> (-0.09%)	⬇️
Tests/SystemTests.cs	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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