Refactor/ssh authentication

[heiytor]

We are introducing a new method for initiating and authenticating requested sessions. This update consolidates the associated pipeline within a New function and auxiliary helpers.

During the authentication process, a session may fail, prompting the client to retry the connection. There are two common scenarios:

1. Providing an incorrect password.
2. Failing to authenticate with the public key and attempting authentication with a password subsequently.

To address this, we have implemented a new Memento pattern capable of efficiently saving and recalling session snapshots. This approach mitigates redundant operations, such as repeated GET requests to the agent device. Additionally, it replaces the old metadata package. Instead of storing small pieces that comprise a session, we now have the capability to preserve the entire session. As a result, we are removing this package. Currently, we need to save two states of a session:

1. When we create it, as calling New for the first time will create a session, subsequent calls must retrieve the already created.
2. When we register it, attempting to register the same session more than once will result in a 409 error in the API.

To illustrate a practical use of this pattern: Imagine a client that wants to connect to a specified device. It first calls the PublicKeyHandler with a specified context. At this stage, there are no sessions associated with the provided context, and a new one will be created. If it fails, the same client (and consequently the same context) will call the PasswordHandler, which also calls session.New. Since we have already created a session in the previous authentication attempt, instead of repeating all operations, we can safely retrieve the same session again but attempt authentication with a password this time.

To align with these adjustments, the register and authentication processes are now invoked outside of NewAgentConnection, which is now solely responsible for establishing a connection to the agent. This method has been renamed to connectAgent and merged with NewAgentConnectionConfiguration. Additionally, ClientConfigurationAuthenticationPublicKey and ClientConfigurationAuthenticationPassword have been replaced with AuthPublicKey and AuthPassword, respectively, for simplification purposes.

The upcoming steps can utilize the Memento to reuse the session.

Additionally, a new sshconf struct has been introduced. sshconf serves as a global variable responsible for managing all immutable environment variables, enabling convenient access to environment configurations. Since these variables are guaranteed to remain constant, it is deemed safe to declare sshconf as a global variable.