Add unused QuicSession interfaces to initiating IETF connection migration: MigratePath(), ValidatePath(). These new interfaces are used by toy client in e2e tests.

Add QuicPathValidator into QuicConnection to do IETF path validation.

Behavior changes:
Cancel path validation if there is any during connection close;

Don't handle socket write error if sending PATH_CHALLENGE on the alternative socket or if the package sent to the alternative peer address exceeds path MTU;

Propagate PATH_RESPONSE to path validator to complete path validation.

Added and modified end-to-end tests to demonstrate connection migration with path validation works on client side.

Protected by quic_reloadable_flag_quic_pass_path_response_to_validator.

PiperOrigin-RevId: 343304329
Change-Id: I53dc52e82fbb9c69f5d2e90e5c90e1be45bb7bb6