KTOR-6759 Darwin: Throw specific exception for SSL Pinning failure

[osipxd]

Subsystem
Client, Darwin

Motivation
KTOR-6759 Darwin: Ambiguous DarwinHttpRequestException for SSL Pinning failure

The problem is that we call completionHandler with NSURLSessionAuthChallengePerformDefaultHandling on SSL pinning failure and then third-party API throws an exception. There are no mechanisms to attach additional data to this exception (or at least I don't know any).
The place where the actual error occurs and the place where DarwinHttpRequestException is constructed are two completely different places.

Solution
Save actual failure thrown during challenge handling to aDarwinTaskHandler and throw it later instead of creating a generic one. Throw specific exception CertificatePinner to make it possible to catch pinning failures specifically:

try {
    client.get("/some-api-with-pinning")
} catch(cause: CertificatePinnerException) {
    println("SSL pinning failure detected.")
}

Naming, comments, and other stylistic things aren’t final yet, but I need this draft to be reviewed.

[osipxd]

I have concerns regarding this exception:

• Previously DarwinHttpRequestException was thrown for such cases, so it is potentially breaking change
• This exception is used for any error thrown by the pinner. Should we give a possibility to distinct between these errors?
• Should we provide a multiplatform exception to be able to handle SSL pinning problems in a unified manner?

[e5l]

Looks good to me.

• I would still consider this as a bug fix
• We can make this exception open, and wait if there are use cases for that.
• We could introduce this exception in ktor-network common code.

Let's target to 3.1.0 and do it

[theextremeprogrammer]

@osipxd Thanks for this PR! I have very little context on the Ktor code, so I don't have much feedback to share. I can see that the exception contains the output message which is exactly what I was hoping to be able to access. I also appreciate how you included some tests. Thanks for your help and @e5l thank you for your review as well! 🙏

[e5l]

Looks good to me.

• I would still consider this as a bug fix
• We can make this exception open, and wait if there are use cases for that.
• We could introduce this exception in ktor-network common code.

Let's target to 3.1.0 and do it

[theextremeprogrammer]

@osipxd Thanks for this PR! I have very little context on the Ktor code, so I don't have much feedback to share. I can see that the exception contains the output message which is exactly what I was hoping to be able to access. I also appreciate how you included some tests. Thanks for your help and @e5l thank you for your review as well! 🙏