Add helper collect method to collect a string up to a sequence

[dimitribouniol]

extension AsyncIteratorProtocol where Element == UInt8 {
    public mutating func collect(
        utf8: String.Type,
        upToExcluding termination: StringProtocol,
        throwsIfOver maximumBufferSize: Int
    ) async throws -> String? {
        let bytes = try await iterator.collect(upToExcluding: termination.utf8, throwsIfOver: throwsIfOver))
        return bytes.map { String(decoding: $0, as: UTF8.self) }
    }
}