destination: add UriLikeIdentity and server_name (#285)

Changes the `TlsIdentity` type in the destination API such that: 

- we add an extra `UriLikeIdentity` identity type that should contain identities that are in URI format (e.g. SPIFFE)
- we add a `server_name` to the `TlsIdentity` type. This allows us to differentiate between an SNI value and a TLS Id value. This is mainly needed because in certain identity systems (SPIFFE/SPIRE) the TLS SAN can be in URI form. A URI cannot be used as a SNI extension in a `ClientHello`, so an alternative SNI value needs to be provided. This brings the need to distinguish between these two concepts. 

For context: 
linkerd/linkerd2-proxy#2506

Signed-off-by: Zahari Dichev <[email protected]>
Co-authored-by: Oliver Gould <[email protected]>

Author: zaharidichev