feat(agent): add mediation grant message

Fixes ATL-3022

prism-agent/src/commonMain/kotlin/io/iohk/atala/prism/walletsdk/prismagent/protocols/MediationGrant.kt

@@ -0,0 +1,36 @@
+package io.iohk.atala.prism.walletsdk.prismagent.protocols
+
+import io.iohk.atala.prism.apollo.uuid.UUID
+import io.iohk.atala.prism.domain.models.Message
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.decodeFromString
+import kotlinx.serialization.json.Json
+
+sealed class MediationProtocolError : Throwable() {
+    class InvalidMediationGrantError : MediationProtocolError()
+}
+
+class MediationGrant {
+    @Serializable
+    data class Body(var routingDid: String)
+
+    var id: String
+    var type = ProtocolType.DidcommMediationGrant.value
+    var body: Body
+
+    constructor(
+        id: String = UUID.randomUUID4().toString(),
+        body: Body
+    ) {
+        this.id = id
+        this.body = body
+    }
+
+    constructor(fromMessage: Message) {
+        if (fromMessage.piuri != ProtocolType.DidcommMediationGrant.value) {
+            throw MediationProtocolError.InvalidMediationGrantError()
+        }
+        this.id = fromMessage.id
+        this.body = Json.decodeFromString(fromMessage.body)
+    }
+}

prism-agent/src/commonMain/kotlin/io/iohk/atala/prism/walletsdk/prismagent/protocols/MediationRequest.kt

@@ -4,7 +4,7 @@ import io.iohk.atala.prism.apollo.uuid.UUID
 import io.iohk.atala.prism.domain.models.DID
 import io.iohk.atala.prism.domain.models.Message
 
-class MediationRequest(
+sealed class MediationRequest(
     val id: String,
     val type: String = ProtocolType.DidcommMediationRequest.value,
     val from: DID,