ConnectionFactory Authmechanisms has incorrect sharing between instances

[evolvedlight]

Describe the bug

if you connect to two RabbitMq instances, one with EXTERNAL auth, one plain, and change the Auth Mechanisms, the instances change each other.

Reproduction steps

1. Create new project with rabbitmq client dependency (latest version)
   Paste the following:

ConnectionFactory factory = new ConnectionFactory();
factory.AuthMechanisms.Clear();
ConnectionFactory factory2 = new ConnectionFactory();
Console.WriteLine(factory2.AuthMechanisms.Count);

Expected behavior

I expect to have a single Plain Auth mechanism
Instead I have Zero auth mechanisms

Additional context

It's because the DefaultAuthMechanisms is referenced by reference rather than making a copy of it.
This bug is triggered by MassTransit: https://github.com/MassTransit/MassTransit/blob/15c44ca0c26e9dfcedbdf3e770621c5f3ceb6975/src/Transports/MassTransit.RabbitMqTransport/RabbitMqTransport/RabbitMqAddressExtensions.cs#L65

[evolvedlight]

I plan to open a PR for this on the weekend, let me know if you don't consider this a bug.

[lukebakken]

Good catch! Thank you for reporting this. A PR is very welcome. If you'd like to base it on the 6.x branch I will then forward-merge your changes. main has diverged quite a bit.