telemetry/gnmi-writer: collect ISIS global state and overload bit

[snormore]

Summary of Changes

• Add isis_global_state extractor and ClickHouse migration capturing per-network-instance ISIS instance name, NET, and level capability from gNMI.
• Add isis_overload_bit extractor and ClickHouse migration capturing the ISIS LSP overload bit per network instance.
• Register both extractors in DefaultExtractors and define *_latest views (latest row per device) on each new table, matching the existing pattern for other gNMI-sourced tables.

Brings ISIS data collected by gnmi-writer closer to parity with the ISIS telemetry already shipped to S3, so we can eventually migrate ISIS consumers off the S3 pipeline onto this one.

Diff Breakdown

Category	Files	Lines (+/-)	Net
Core logic	1	+72 / -0	+72
Scaffolding	3	+100 / -0	+100
Total	4	+172 / -0	+172

Pure addition: one extractor file with the new gNMI-walking logic, plus record structs and SQL migrations declaring the storage shape.

Key files (click to expand)

• telemetry/gnmi-writer/internal/gnmi/extractors.go — adds extractIsisGlobalState and extractIsisOverloadBit and registers them in DefaultExtractors.
• telemetry/gnmi-writer/internal/gnmi/records.go — adds IsisGlobalStateRecord and IsisOverloadBitRecord with their ClickHouse TableName() mappings.
• telemetry/migrations/20250410000000_isis_global_state.sql — new isis_global_state table (30-day TTL, MergeTree) plus isis_global_state_latest view.
• telemetry/migrations/20250410000001_isis_overload_bit.sql — new isis_overload_bit table (30-day TTL, MergeTree) plus isis_overload_bit_latest view.

Testing Verification

• gnmi-writer unit tests and `go vet` pass on the affected package.
• Migrations apply cleanly against a local ClickHouse via `RunMigrations`.