net: lwm2m: Add support for X509 certificates

Add support for using X509 certificates.
Default settings use ECDSA certificates with SHA256 hash.

When different settings are required clients should overwrite
struct lwm2m_ctx->load_credentials() and
struct lwm2m_ctx->set_socketoptions()

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>