[meta] Add opt-in saithrift RPCs for draining SAI notifications

[nicholasching]

Context / motivation
Part of the SAIVPP unit-test framework. SAI delivers port state and BFD session state asynchronously through function pointers written into SAI_SWITCH_ATTR_PORT_STATE_CHANGE_NOTIFY and SAI_SWITCH_ATTR_BFD_SESSION_STATE_CHANGE_NOTIFY. A Thrift client cannot supply a function pointer, so there is currently no way for any saithrift-based test suite to observe that a notification was delivered. This blocks testing the notification path on any target, not just VPP.

What this change does

• meta/templates/sai_thrift_utils.tt, meta/templates/sai.thrift.tt. Declare sai_thrift_enable_notifications() and sai_thrift_drain_notifications(), plus a sai_thrift_notification_event_t record carrying notification type, object id and state.
• meta/templates/sai_rpc_server_functions.tt. Add the two method names to the sai_utils_functions regex so the generator emits stub bodies instead of trying to bind them to SAI metadata.
• meta/sai_rpc_frontend.cpp. Implement both methods in sai_rpcHandlerFrontend, following the existing sai_thrift_object_type_get_availability pattern. The enable RPC installs server-owned callbacks and is idempotent; the drain RPC is non-blocking and atomically empties a mutex-protected bounded queue.
• Post-create registration. The callbacks are installed on the already-created switch via set_switch_attribute, not at create time. SwitchStateBase::set_switch_default_attributes() writes NULL into both attributes during initialize_default_objects, so a pointer supplied at create time is silently discarded. Both attributes are CREATE_AND_SET and the senders re-read them when an event fires, so a post-create set is correct, simpler, and additionally suppresses the forced port-UP notifications raised during host-interface creation.

Scope / risk

• Default-preserving: behaviour is unchanged unless a test explicitly calls the enable RPC. No existing RPC is modified.
• No function pointer crosses Thrift. The callbacks live entirely inside saiserver; only notification type, object id and state are serialized.
• Bounded: the queue holds 256 events and drops the oldest on overflow, so a client that never drains cannot grow memory without limit.
• RAII containers and standard synchronization only. No new dependencies, no unsafe C memory or string calls, no credentials or certificates.
• No SAI headers, metadata definitions, or backend behaviour change.
• Validated behavior: gensairpc.pl completes and the generated service contains both methods; saithriftv2 and saiserver build and link; enable followed by drain from a throwaway client returns an empty list without crashing. End-to-end validation is in the notification test PR.

Dependencies
None. This is a prerequisite for the notification test cases, #2336.

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[yue-fred-gao]

can we define enum type for notification type?

[nicholasching]

Hi @kcudnik @tjchadaga, when you have a chance, could you please invoke azure pipelines to run checks. Thanks!

[yue-fred-gao]

lgtm