ByteBuffer should have hex bytes initialiser / write method

[weissi]

This is pretty much a companion of #2447 / #2475. The aforementioned bugs/patches introduce the ability to get the hex dump from a ByteBuffer which is awesome.

Next, I'd like to construct/write into a ByteBuffer from a hex dump:

• ByteBuffer(hexEncodedBytes: "68 65 6c 6c 6f 20 77 6f 72 6c 64 0a")
• buffer.writeHexEncodedBytes("68656c6c6f20776f726c640a")

I'd say it just needs to support the .plain format (i.e. 2 hex-digit values with whitespace ignored)

---

Super basic test:

var buffer = ByteBuffer(hexEncodedBytes: "68 65 6c 6c 6f 20 77 6f 72 6c 64 0a")
buffer.writeHexEncodedBytes("68656c6c6f20776f726c64")
buffer.writeHexEncodedBytes("")
buffer.writeHexEncodedBytes("0a")
XCTAssertEqual(ByteBuffer(string: "hello world\nhello world\n"), buffer)

another one:

var allBytes = ByteBuffer()
for x in UInt8.min ... UInt8.max {
    buffer.writeInteger(x)
}

let allBytesHex = allBytes.hexDump(format: .plain)
let allBytesDecoded = ByteBuffer(hexEncodedBytes: allBytesHex)

XCTAssertEqual(allBytes, allBytesDecoded)

[ali-ahsan-ali]

I have taken a look and would like to pick this up after https://github.com/apple/swift-nio/issues/2734.