[FEATURE] Add GreptimeDB datasource Plugin

[sunchanglong]

Description

This pull request adds a new GreptimeDB plugin to the plugins repository so Perses can query and visualize metrics & logs data from GreptimeDB.

The implementation covers the main GreptimeDB query workflows used in Perses dashboards:

querying time series data from GreptimeDB based on panel time range
converting GreptimeDB responses into the data format expected by Perses panels, so charts and related visual components can consume results without additional handling

A few points worth calling out for review:

GreptimeDB response data is normalized and mapped into Perses-compatible series/frames, including timestamps, values, and label dimensions
plugin schemas, SDK integration helpers, datasource configuration, query editor support, and tests are included in the same change
only files required for the GreptimeDB plugin and workspace/plugin registration were touched
This change is intended to address GreptimeDB datasource support discussed in the linked issue and related follow-up conversations.

Screenshots

Checklist

• Pull request has a descriptive title and context useful to a reviewer.
• Pull request title follows the [<catalog_entry>] <commit message> naming convention using one of the
  following catalog_entry values: FEATURE, ENHANCEMENT, BUGFIX, BREAKINGCHANGE, DOC,IGNORE.
• All commits have DCO signoffs.

UI Changes

• Changes that impact the UI include screenshots and/or screencasts of the relevant changes.
• Code follows the UI guidelines.