prepare release 2.4.0

[Slach]

Enhancement:

• Add support for Grafana 8.x unified alerts, fix New [Unified Alerts] API is broken #380
• Add TLS support for backend alerts part of plugin Clickhouse Alert Throwing Error - request handler error: plugin unavailable #356 (comment)
• Add $naturalTimeSeries macro, look details in https://github.com/Vertamedia/clickhouse-grafana/pull/89/files#diff-cd9133eda7b58ef9c9264190db4534a1be53216edbda9ac57256fbd800368c03R383-R412
• Update golang-plugin-sdk-go to latest version
• Properly format Value in Table format, look details toTable: format value #379
• Remove toDateTime64 casting for column when time column is already DateTime64 to improve performance. Change test to ensure the casting is removed from the query, fix remove toDateTime(xx,3) to improve performance after fix clickhouse issue #360
• implements $timeFilter64ByColumn(column_name) macro, fix $timeFilterByColumn not working with DateTime64 #343

Fixes:

• implements properly GET and POST support for alert queries, fix clickhouse-grafana falls back to POST for alert queries #353
• SQL syntax highlight now works always, fix /mode-clickhouse.js request #174, fix the SQL highlighted is not supported for Grafana 7.x #381
• SQL syntax highlight, fix /mode-clickhouse.js request #174, fix the SQL highlighted is not supported for Grafana 7.x #381
• fix Alerts break authentication to Clickhouse #376,
• fix negative behavior for $perSecondColumns Support for non-negative rates in $perSecondColumns #337
• fix [Enhancement] in SQL query, treat -- inside quotes not as comment #374, ignore -- inside quotas as comment