xds: change clusterimpl to add SNI to handshake info (#9016) This PR is the final PR for implementation of [gRFC A101](https://github.com/grpc/proposal/blob/master/A101-SNI-setting-and-SNI-SAN-validation.md) This PR does the following : - Change NewSubconn in ClusterImpl to add the hostname to attribute of the address so that it can be propagated to the ClientHandshake and retrieved there to decide the SNI which can be either DNS Hostname or endpoint Hostname if `AutoHostSni` is set , or the SNI received from control plane. This is done because each endpoint can have a different hostname. It is not a config that can be used across all endpoints of the cluster , so it cannot be set in handshake info. - Add `AutoHostSni` filed to the handshake info. - Adds a functions to set and get hostname from address attributes. As of now, there was a function to get the Hostname from address balancer.Attributes and set in endpoint atrributes. - Adds E2E tests to verify the complete SNI setting and validation flow. - Fix comments that mention CDS balancer creates handshake info. Note: We will turn the environment variable to true only after inter-op tests pass. RELEASE NOTES: - xds: add SNI support and SAN validation behind GRPC_EXPERIMENTAL_XDS_SNI ([gRFC A101](https://github.com/grpc/proposal/blob/master/A101-SNI-setting-and-SNI-SAN-validation.md))
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