Delegate the cookie signing to a Signer component.

Add a new concept of "signer" in order to delegate the process of cookie signing, and cookie signature check.

The cookie filter will no more do this work, but delegates signing and signature verification to its injected signer.

Create a DefaultCookieSigner component which use the existing SignatureKey in order to stay compatible with previous releases.
The DefaultCookieSigner get the signature key by dependency injection and use the HMAC-SHA1 algorithm (defined in Crypto class) to sign the cookie.

I wasn't sure about the fact of giving a name to the injected Signer in RestxSessionCookieFilter. But as I think an application might have multiple signers, I defined it.
For the same reason the signature injected in DefaultCookieSigner might be named too ?!