[Ingest Manager] Reduce height of Agent configuration selection box

[mostlyjason]

We've gotten feedback from several people including Ron, Ruflin and Shay that the Agent configuration selection box on the integration configuration page takes up too much vertical space. This is particularly true when a user is just getting started and there is only a single item in the list. This results in a large amount of white space. It also pushes the content that the user needs to configure down the page.

Let's change it to display a single item instead of multiple when the item is selected. We can offer a dropdown search box that allows users with 100s of configurations to filter them.

One option:
https://elastic.github.io/eui/#/forms/combo-box but limited to a single item

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[mostlyjason]

From Ron on Slack:

I see. EuiComboBox works for now 👍
If you agree it would be nice to have a regular dropdown with search, do you mind bringing the ask for search for SuperSelect to EUI?

[hbharding]

Design update

Loom recording

We can change step 1 "Select an agent configuration" to use a EuiComboBox that only accepts 1 value. This component allows the user to search for configurations, and it supports custom renderings in the dropdown menu.

Some details to note:

• This select box should be pre-populated with the Default agent configuration.
  • In the case that configurations can only have 1 instance of an integration (like Endpoint), we can't pre-select the default agent config if the integration already exists. We can show the agent configuration as disabled in the dropdown menu.
• In the dropdown, I added the word "enrolled" to the agent count to clarify that these are agents enrolled with fleet (i.e. doesn't include standalone agents).
• I moved the "create agent configuration" to the top right of the select input to reduce overall vertical height
• I'm showing the # of enrolled agents as help text below the combo box. This # should change if the user selects a different config.