[FEATURE] Add OpenSearch datasource and log query plugin with PPL support

[Oliver-ke]

Description

Adds an OpenSearch datasource + log query plugin

These changes includes:

• Plugin scaffold — OpenSearchDatasource + OpenSearchLogQuery plugins (TypeScript, React, Module Federation via rsbuild).
• PPL client (opensearch-client.ts) - POSTs to /_plugins/_ppl, handles relative datasource URLs, throws a typed OpenSearchPPLError(status, body) on non-200.
• Time bounding + row mapping (get-opensearch-log-data.ts) - wraps user PPL with a where clause on the configured timestamp field; maps rows to LogEntry and surfaces
  remaining columns (e.g. traceId) as labels.
• Field-name overrides - timestampField / messageField on the query spec for indexes that don't use @timestamp / message (e.g. OpenTelemetry's body).
• Editor (OpenSearchLogQueryEditor.tsx) - datasource picker, index pattern, timestamp/message field overrides, PPL textarea, and an inline Alert driven by an optional
  queryError prop (parses error.reason / error.details from the OpenSearch error JSON).
• Trace-to-logs pivot - relies on the existing replaceVariables path, so a $traceId dashboard variable substitutes into the PPL before send. Example dashboard at
  opensearch/docs/examples/trace-to-logs.json (Tempo + OpenSearch; Jaeger swap documented).
• CUE schema (schemas/queries/opensearch-log-query/query.cue) - mirrors the TS spec including the new optional fields.
• Go SDK (sdk/go/query/log) - PluginSpec + TimestampField() / MessageField() options for dashboards-as-code.
• Tests - 26 TS tests (PPL bounder, row mapper with overrides + fallbacks, $traceId substitution + dependsOn, PPL client URL/headers/error, editor inputs + error alert)
  and 2 Go tests (builder JSON shape + omitempty for optional fields).

Screenshots

[TBD]

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.