Implement adaptive relay

[gnarea]

Per https://specs.relaynet.link/RS-017

See also: AwalaApp/specs#68

[gnarea]

Here's a working example of registering + delivering parcels + collecting parcels from a public gateway:

import kotlinx.coroutines.delay
import kotlinx.coroutines.flow.map
import kotlinx.coroutines.flow.take
import kotlinx.coroutines.flow.toList
import kotlinx.coroutines.runBlocking
import org.junit.jupiter.api.Test
import tech.relaycorp.relaynet.bindings.pdc.Signer
import tech.relaycorp.relaynet.bindings.pdc.StreamingMode
import tech.relaycorp.relaynet.issueEndpointCertificate
import tech.relaycorp.relaynet.issueParcelDeliveryAuthorization
import tech.relaycorp.relaynet.messages.Parcel
import tech.relaycorp.relaynet.messages.control.PrivateNodeRegistrationRequest
import tech.relaycorp.relaynet.wrappers.generateRSAKeyPair
import kotlin.test.assertEquals

class RealTest {
    private val localGatewayPort = 8082

    @Test
    fun test_connection(): Unit = runBlocking {
        val privateGatewayKeyPair = generateRSAKeyPair()
        val client = PoWebClient.initLocal(localGatewayPort) // <-- TODO: Replace with `initRemote()`
        client.use {
            val registrationRequest = client.preRegisterNode(privateGatewayKeyPair.public)
            val registration =
                client.registerNode(registrationRequest.serialize(privateGatewayKeyPair.private))

            val recipientKeyPair = generateRSAKeyPair()
            val recipientCertificate = issueEndpointCertificate(
                recipientKeyPair.public,
                privateGatewayKeyPair.private,
                registration.privateNodeCertificate.expiryDate,
                registration.privateNodeCertificate,
                registration.privateNodeCertificate.startDate
            )
            val senderKeyPair = generateRSAKeyPair()
            val senderCertificate = issueParcelDeliveryAuthorization(
                senderKeyPair.public,
                recipientKeyPair.private,
                recipientCertificate.expiryDate,
                recipientCertificate,
                recipientCertificate.startDate
            )

            val parcel = Parcel(
                recipientCertificate.subjectPrivateAddress,
                ByteArray(0),
                senderCertificate,
                senderCertificateChain = setOf(
                    recipientCertificate,
                    registration.privateNodeCertificate
                )
            )

            val privateGatewaySigner =
                Signer(registration.privateNodeCertificate, privateGatewayKeyPair.private)
            client.deliverParcel(
                parcel.serialize(senderKeyPair.private),
                privateGatewaySigner
            )

            delay(3_000)

            val parcelsCollected = client.collectParcels(
                arrayOf(privateGatewaySigner),
                StreamingMode.KeepAlive
            ).map {
                it.ack()
                it.deserializeAndValidateParcel()
            }.take(1).toList()
            assertEquals(1, parcelsCollected.size)
            assertEquals(parcel.id, parcelsCollected.first().id)
        }
    }
}

And here's the documentation on how to write unit tests with a mock client: https://github.com/relaycorp/relaynet-jvm-testing#mock-pdc-client