Add Support of RPK at Client side

[sbernard31]

This aims to implements #435.
This PR need the eclipse-californium/californium#536.

To use it with this PR.

# get californium credential_store branch
git clone git@github.com:eclipse/californium.git
cd /in/your/californium/directory
git checkout credential_store

# change version 
mvn versions:set -DnewVersion=2.0.0-credStore  

# install it
mvn clean install -DskipTests

# if you are using eclipse
cd /in/your/leshan/directory
mvn eclipse:eclipse

[sbernard31]

@rkimsb2 you should be interested in this.

[sbernard31]

It seems this is not the right way if we want to support co-located server. (see eclipse-californium/californium#536 for more details)
I don't know if this make sense to try to support this use case, but I will finally go back to the #450 way.

I don't close yet this PR, so if you have any feedback/thought about this you're welcome !

[sbernard31]

Deprecated by #546