fix: Only call newExecutionClient if needed

[erezrokah]

Summary

This is more of hot fix/patch to #610.
#610 had an unwanted side effect on the migrate command, as now we init the plugin before running it.
initializing the plugin is expensive, requires credentials and does a whole lot of validations, for example:
https://github.com/cloudquery/cloudquery/blob/a06139efad9a2e8de14f186a223222c9d52ce58a/plugins/source/aws/client/client.go#L383
https://github.com/cloudquery/cloudquery/blob/a06139efad9a2e8de14f186a223222c9d52ce58a/plugins/source/azure/client/client.go#L124

Those seem unnecessary for tables migration (unless one uses dynamic tables).

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[github-actions]

⏱️ Benchmark results

• DefaultConcurrencyDFS-2 resources/s: 11,819
• DefaultConcurrencyRoundRobin-2 resources/s: 12,379
• Glob-2 ns/op: 244.5
• TablesWithChildrenDFS-2 resources/s: 24,288
• TablesWithChildrenRoundRobin-2 resources/s: 27,867
• TablesWithRateLimitingDFS-2 resources/s: 28.4
• TablesWithRateLimitingRoundRobin-2 resources/s: 814.8
• BufferedScanner-2 ns/op: 12.29
• LogReader-2 ns/op: 36.4

[yevgenypats]

Looks good. Seems we need to add then the same workaround for GetDynamicTables. and use GetTablesForSpec in GetDynamicTables. Im also not a fan of this fix but I guess that's fine for now. basically it doesn make sense to authenticate to migrate in a way but yeah if dynamic_tables is nil then in theory this shouldn't be needed so that's the only workaround possible actually

[erezrokah]

Seems we need to add then the same workaround for GetDynamicTables. and use GetTablesForSpec in GetDynamicTables

Can you explain this a bit? Not sure I got it, sessionTables is still initialized by Init even if you don't have dynamic tables

[yevgenypats]

I see. I think it's good then. I missed it in the GitHub UI, sorry.