Add handler in bmcctld for RACK_MANAGER_DATA_TABLE for rack manager telemetry

[judyjoseph]

Description

bmcctld now subscribes to RACK_MANAGER_ALERT and gates Switch-Host power-on while any alert reads CRITICAL (has_critical_rack_mgr_alert). Because Rack Manager only writes deviations (CRITICAL/MAJOR/MINOR) to RACK_MANAGER_ALERT and never writes NORMAL, a raised alert never clears itself.

Per the pmon-bmc-design document, Rack Manager also publishes periodic telemetry to a separate STATE_DB table, RACK_MANAGER_DATA, whose purpose is to signal when a critical alert has cleared. This PR adds a handler that consumes that telemetry. When a sensor's telemetry severity returns to NORMAL, the corresponding RACK_MANAGER_ALERT entry is deleted so the power-on gate no longer treats it as active.
REF : https://github.com/sonic-net/SONiC/blob/master/doc/bmc/sonicBMC/pmon-bmc-design.md#2121-db-schema

Motivation and Context

-- Add RACK_MANAGER_DATA subscriber and integrate it into the select loop and file-descriptor-to-handler dispatch map.
-- Add _handle_rack_mgr_data(key, fvs):
-- Read severity from:
severity field for temperature, flow, and pressure telemetry.
leak field for Rack_level_leak.
-- Treat non-NORMAL telemetry as informational only; no action is taken.
Alerts continue to be raised via RACK_MANAGER_ALERT.
-- On NORMAL: Delete only the matching RACK_MANAGER_ALERT| entry (per-key scope).
Clear the cached last-severity value for that key.
Log a notice.
No-op if no active alert exists for that key.

Add RACK_MGR_TELEMETRY_KEYS module constant containing the four supported telemetry sensors:
Inlet_liquid_temperature
Inlet_liquid_flow_rate
Inlet_liquid_pressure
Rack_level_leak

How Has This Been Tested?

Following unit tests are added

   1. test_data_subscriber_registered - the rack-manager data subscriber is registered and routed to _handle_rack_mgr_data.               ┃
   2. test_normal_telemetry_clears_active_alert - NORMAL telemetry deletes the matching alert row and resets dedup state.                 ┃
   3. test_normal_telemetry_no_active_alert_is_noop - NORMAL telemetry with no active alert does nothing.                                 ┃
   4. test_non_normal_telemetry_leaves_alert - non-NORMAL telemetry never clears an alert.                                                ┃
   5. test_rack_level_leak_telemetry_clears_via_leak_field - Rack_level_leak reports NORMAL via the 'leak' field and clears the alert.    ┃
   6. test_mixed_case_normal_clears - a mixed-case "Normal" severity still clears the alert (case-insensitive).                           ┃
   7. test_unknown_telemetry_key_ignored - telemetry under a key outside ALERT_KEYS is ignored.       

Additional Information (Optional)

[mssonicbld]

/azp run

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[mssonicbld]

This PR has backport request label(s) for branch(es): msft-202608, but is missing required test information. Please make sure you tick the tested branch(es) in the Tested branch section and provide test evidence (e.g., 202608: <test result>) in the Test result section as well in your PR description.

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