[Discover] Support selecting multiple datasets

[joshuali925]

Is your feature request related to a problem? Please describe.

Currently discover only supports selecting single dataset (ie. single index-pattern, single index, etc). It would be useful to support multi-select.

Describe the solution you'd like

Basic implementation for multi-select is added in #8255, but it takes a shortcut by combining selected datasets into one dataset after user configuration. Discover still keeps one reference of dataset. This has some limitations such as we cannot display fields separately per dataset, and the implementation hard codes the logic when combining datasets. It should be more flexible. See #8255 (comment)

Additionally multi-select component should display proper error message when incompatible datasets are selected together, see #8255 (comment)

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Additional context

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[joshuali925]

#8494 should fix this:

the implementation hard codes the logic when combining datasets