Add X509 support to Leshan Client

[sbernard31]

Security parameters can change dynamically at bootstrap, so several solutions :

1. add to scandium a kind of interface like PskStore for X509 certificate.
2. restart create a new DTLS connector if security parameters change.

[AlexITC]

if you are interested, we already support client certificates in our fork: https://github.com/artikcloud/leshan

[sbernard31]

Thx @AlexITC, I will look at this.

Currently, I hesitate between the 2 solutions exposed above.

Solution 1. sounds better to me but this needs Californium modification.
That's why I begin to implement the 2. solution in #450.
But @boaks start to work on a very interesting way to handle 1. in Californium.
So I waiting to know where to go :).

[sbernard31]

It should be fixed by #565.