alts: add close to TsiHandshaker to avoid resource leak for some impls

[creamsoup]

Some handshaker implementation has internal closable resources. It only works if everything goes well (current implementation relying on the internal resource closes itself).

In failure case, it may cause leak depends on the implementation of tsi handshaker. Having close in TsiHandshaker and TsiHandshakeHandler explicitly close the handshaker should avoid such leak; less error prone.