[FEA] Mark `DataFrame.insert` as `_external_only_api`

[vyasr]

Is your feature request related to a problem? Please describe.
DataFrame.insert is an expensive operation because it triggers index equality checking and potentially index realignment, both of which are typically unnecessary when this function is used internally because the items are either already aligned or we know exactly what needs to be done to align them.

Describe the solution you'd like
DataFrame.insert should be decorated with _external_only_api, and any code using it should be converted to use _insert.