Remove socks5 authentication check from the TransportStrategy layer

[mojganii]

We don't necessarily need a switch-case for SOCKS authentication that involves retrieving credentials in the TransportStrategy when we can achieve the same outcome using an optional value for the authentication part. This pull request transforms the SocksAuthentication enum into a struct and makes it an optional variable in SocksConfiguration.

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This change is 

[rablador]

Reviewed 6 of 6 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @mojganii)

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ios/MullvadVPN/Coordinators/Settings/APIAccess/Models/PersistentProxyConfiguration+ViewModel.swift line 24 at r1 (raw file):

        )

        socks.username = config.authentication?.username ?? ""

Nit: I know that username and password are "" by default, but perhaps we could do something like the snippet below in order to make it clearer that we're only assigning username and password when we actually have authentication.

Code snippet:

config.authentication.flatMap { authentication in
    socks.username = authentication.username
    socks.password = authentication.password
}

[rablador]

Reviewed 2 of 6 files at r2, all commit messages.
Reviewable status: 2 of 6 files reviewed, 1 unresolved discussion (waiting on @mojganii)

[rablador]

Reviewed 4 of 6 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved

[buggmagnet]

Reviewed 6 of 6 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved