Description of problem:

Plugins defined in include files are overwriting plugin configuration as a whole. They should instead merge, only overwriting the specific keys they define while retaining the keys defined in previous include files, as is being done for plugins under plugins key.

This is needed for Extensions plugin to be able to use pluginConfig from default plugins file when enabling preinstalled plugins.

Prerequisites (if any, like setup, operators/versions):

Steps to Reproduce

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Actual results:

Expected results:

Reproducibility (Always/Intermittent/Only Once):

Build Details:

Additional info (Such as Logs, Screenshots, etc):