Protobuf Java and Kotlin code generation. (#64)

Co-authored-by: Stian Brandt <stian.brandt@it-smurfan.no>

api/kotlin/README.md

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+# chirpstack-api
+
+ChirpStack gRPC API message and service wrappers for Kotlin.
+
+## Install
+
+An artifact is not yet public on Maven Central. You can build your own jar with
+
+```sh
+make jar
+```
+
+or install the artifact in your local Maven repository `$HOME/.m2` with
+
+```sh
+make install
+```
+
+## Usage
+
+All messages, services, constants, etc. are auto-generated from the ChirpStack protobuf definitions. The result is that
+this package structure matches that of the protobuf definitions.
+
+The protobuf definitions can be found here: https://github.com/chirpstack/chirpstack/tree/master/api/proto
+
+### Example
+
+#### Maven
+
+```xml
+<dependency>
+  <groupId>io.chirpstack</groupId>
+  <artifactId>chirpstack-api-kotlin</artifactId>
+  <version>1.0-SNAPSHOT</version>
+</dependency>
+```
+
+#### Gradle
+
+```gradle
+dependencies {
+    implementation("io.chirpstack:chirpstack-api-kotlin:1.0-SNAPSHOT")
+}
+```
+
+#### Example #1: List tenants
+
+Note: To make this example work you will also need to add kotlin coroutines as a dependency
+
+```gradle
+dependencies {
+    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.6.4")
+}
+```
+
+```kotlin
+import io.chirpstack.api.TenantServiceGrpcKt
+import io.chirpstack.api.listTenantsRequest
+import io.grpc.ManagedChannelBuilder
+import io.grpc.Metadata
+import kotlinx.coroutines.runBlocking
+
+fun main() {
+    val channel = ManagedChannelBuilder
+        .forAddress("localhost", 8080)
+        .usePlaintext()
+        .build()
+    val token = "Bearer <INSERT_TOKEN_HERE>"
+    val metadata = Metadata()
+    val key = Metadata.Key.of("authorization", Metadata.ASCII_STRING_MARSHALLER)
+    metadata.put(key, token)
+    val stub = TenantServiceGrpcKt.TenantServiceCoroutineStub(channel)
+    val request = listTenantsRequest {
+        limit=10
+    }
+    val response = runBlocking { stub.list(request, metadata) }
+    println("Number of tenants: ${response.totalCount}")
+}
+```