credential: Re-write the credentialmodel as a tree

The CredentialModel was previously only used for SIP credential. This
commit extend it so it can handle multiple types of Credentials

**WARNING** Introduce a minor API change. CredentialModel::addCredential()
now take a Credential::Type as argument. This will be mitigated by the
next commit that introduce a credential creation model.

Issue: #78102
Change-Id: Id7e1fc9391784ff654dfdfbb812c581fbfa8fd1e