support dynamic IoTDB table-model variables

[CoollZzz]

This pull request adds table-model template variable query support to the IoTDB Grafana plugin.

Changes

• Added support for variable queries using:

  table:<database>:<SQL>
  

• Executed these queries through the native IoTDB table-model RPC client.

• Added validation and error handling for malformed queries, missing database or SQL values, execution failures, and multi-column results.

• Converted valid single-column results into Grafana-compatible string arrays while preserving order and skipping NULL values.

• Preserved existing legacy REST-based variable queries.

Dynamic node discovery

• Added the $__activeFrom macro for table-model variable queries.
• The plugin expands it to the current time minus five minutes, as an ISO 8601 UTC timestamp literal.
• This lets dashboards discover recently active nodes dynamically.
• Nodes that stop producing samples disappear from the variable list after the liveness window, without modifying historical data.

Testing

• Added dependency injection for the table executor and clock to support unit testing without a live IoTDB service.
• go test ./..., go vet ./..., and go build ./... passed.