ChaCha20 Explicitly Set Counter

[btoms20]

Feature: Set Counter in ChaCha20 Cipher's Initializer

Checklist:

• Correct file headers (see CONTRIBUTING.md).
• Formatted with SwiftFormat.
• Tests added.

Changes proposed in this pull request:
Are there any reasons, security or otherwise, that the leading 4 bytes in the ChaCha20 Cipher's counter param can't be explicitly set to something other than [0, 0, 0, 0]?

I'm trying to generate a QUIC header protection mask (as discussed in RFC 9001 - section 5.4.4) and it requires the 4 byte prefix in the counter to be explicitly set upon the cipher's initialization.

The change proposed in this PR is to add a counter param to the ChaCha20 initializer that defaults to the standard 4 zero bytes. The default value prevents breaking changes while keeping only a single initializer. I also wasn't sure what API would be preferable...

1. Default counter param in the current initializer (this way the user knows for sure they're setting the counter).

public init(key: Array<UInt8>, counter: Array<UInt8> = [0, 0, 0, 0], iv nonce: Array<UInt8>) throws {
    precondition(counter.count == 4 && (nonce.count == 12 || nonce.count == 8))
    ...
}

2. Allow for 16 byte Nonce / IV (perhaps more confusing).

public init(key: Array<UInt8>, iv nonce: Array<UInt8>) throws {
    precondition(nonce.count == 16 || nonce.count == 12 || nonce.count == 8)
    ...
}

Either method would allow for the following to be accomplished...
Example from RFC 9001 Appendix A.5 Quic v1

let hpKey = Array(hex: "0x25a282b9e82f06f21f488917a4fc8f1b73573685608597d0efcb076b0ab7a7a4")

///            |  Counter |           IV             |
/// Sample: 0x | 5e5cd55c | 41f69080575d7999c25a5bfb |
let counter = Array(hex: "0x5e5cd55c")
let iv = Array(hex: "0x41f69080575d7999c25a5bfb")

let mask = try CryptoSwift.ChaCha20(key: hpKey, counter: counter, iv: iv).encrypt(Array<UInt8>(repeating: 0, count: 5))

XCTAssertEqual(mask, "aefefe7d03")

Let me know what you think

Thanks

[krzyzanowskim]

it's been a while, I'm sorry. Do you mind resolve conflict here? I'm ok with the change