Add Transport Layer abstraction at Bootstrap server side.

[sbernard31]

This aims to implement #1025 at Bootstrap Server side.

This is is largely inspired by #1318

The API looks like :

// You create your LeshanServer with the builder like before
LeshanBootstrapServerBuilder builder = new LeshanBootstrapServerBuilder();

// But builder does not allow to configure any details about transport layer anymore
// instead you need to provide a LwM2mBootstrapServerEndpointsProvider
// LwM2mBootstrapServerEndpointsProvider will provide 1 or several LwM2mBootstrapServerEndpoint

// For Californium (coap/coaps), Endpoint Provider API looks like this :
// --------------------------------------------------------
CaliforniumBootstrapServerEndpointsProvider.Builder endpointsBuilder = new CaliforniumBootstrapServerEndpointsProvider.Builder(
		    new CoapBootstrapServerProtocolProvider(), new CoapsBootstrapServerProtocolProvider());

// Change Californium configuration :
Configuration californiumConfig = endpointsBuilder.createDefaultConfiguration();
californiumConfig.set(DtlsConfig.DTLS_RECOMMENDED_CIPHER_SUITES_ONLY, true);
endpointsBuilder.setConfiguration(californiumConfig);

// By default the Californium Builder will create 1 endpoint by protocolProvider
// But you can create it manually like this :
endpointsBuilder.addEndpoint(new InetSocketAddress(0), Protocol.COAP);
endpointsBuilder.addEndpoint("coaps://localhost:5684");

// then add the endpoints provider to LeshanServerBuilder
builder.setEndpointsProvider(endpointsBuilder.build());

More tasks for later :

• there is not so much javadoc and so it should be added later.
• there is still some TODO TL : ... to implement.
• Currently we can add only 1 LwM2mEndpointsProvider but we should be able to add several one.
• I think we should have new interface to separate LWM2M request and LWM2M bootstrap request.
• of course code should also be moved to follow new Transport Layer Module design. #1295

---

'Fun' Fact (or not ...)

Coding this one, I faced an issue which makes me crazy ... 🤯
I was unable to compile some code which does not contain any issue at all.

After investigation I find this was a java compiler issue on java 8 : https://stackoverflow.com/questions/34273176/maven-compilation-fails-with-cannot-find-symbol-while-with-eclipse-it-compile

So in BootstrapHandlerTest if import was in this order :

import org.eclipse.leshan.core.response.LwM2mResponse;
import org.eclipse.leshan.core.response.ResponseCallback;
import org.eclipse.leshan.core.response.SendableResponse;
import org.eclipse.leshan.server.bootstrap.BootstrapHandlerTest.MockRequestSender.Mode;
import org.eclipse.leshan.server.bootstrap.request.BootstrapDownlinkRequestSender;
import org.junit.Test;

you get a :

BootstrapHandlerTest.java:[160,54] cannot find symbol
 symbol: class BootstrapDownlinkRequestSender
 location: class org.eclipse.leshan.server.bootstrap.BootstrapHandlerTest

But this below is OK :

import org.eclipse.leshan.core.response.LwM2mResponse;
import org.eclipse.leshan.core.response.ResponseCallback;
import org.eclipse.leshan.core.response.SendableResponse;
import org.eclipse.leshan.server.bootstrap.request.BootstrapDownlinkRequestSender;
import org.eclipse.leshan.server.bootstrap.BootstrapHandlerTest.MockRequestSender.Mode;
import org.junit.Test;

As I can change the order because we check that import are well sorted with maven.
I just moved the Mode enum from BootstrapHandlerTest.MockRequestSender to BootstrapHandlerTest ;