Add mTLS examples ported from akka-grpc PR #1781

[pjfanning]

The changes in https://github.com/akka/akka-grpc/releases/tag/v2.3.2 are now available under the Apache license.

Ports the mTLS (mutual TLS) support from akka-grpc#1781, replacing all akka references with pekko. Uses pekko-pki's DERPrivateKeyLoader/PEMDecoder.

New files

• docs/src/main/paradox/mtls.md — mTLS documentation; wired into index
• plugin-tester-java/ — MtlsGreeterServer.java, MtlsGreeterClient.java, cert files under src/main/resources/certs/
• plugin-tester-scala/ — MtlsGreeterServer.scala, MtlsGreeterClient.scala, MtlsIntegrationSpec.scala, cert files, logback.xml

Build changes

• build.sbt: adds pekko-pki to both plugin testers; adds pekko-actor-testkit-typed (test) to Scala tester
• project/Dependencies.scala: adds Runtime.logback to pluginTester
• plugin-tester-java/application.conf: adds pekko.http.server.enable-http2 = on

SSL context construction (both Java and Scala)

Server loads its own keypair and trusts any client cert signed by the configured CA:

val serverPrivateKey = DERPrivateKeyLoader.load(PEMDecoder.decode(classPathFileAsString("certs/localhost-server.key")))
keyStore.setKeyEntry("private", serverPrivateKey, new Array[Char](0), Array[Certificate](serverCert))
// ...
engine.setNeedClientAuth(true)

Client presents its keypair and trusts the server cert via the same CA:

val clientPrivateKey = DERPrivateKeyLoader.load(PEMDecoder.decode(classPathFileAsString("certs/client1.key")))
val clientSettings = GrpcClientSettings.connectToServiceAt("localhost", 8443).withSslContext(sslContext())