[Fleet] Allow to reassign agent configuration to agents

[nchaulet]

Description

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Update: We're not supporting bulk actions for Alpha 1. Users can reassign an agent by clicking the icon in the "actions" column, or on the agent detail page under the "actions" button. Upon reassignment, show a success toast that says title: (check icon) Successfully reassigned "host name" to "name of config" agent configuration.

Users can reassign agent configuration to a single agent or multiple agents by using bulk actions in Fleet. After the user reassigns the agents, they should see a success toast message saying title: "(check icon) Reassigned <#> agent(s) to use "name of config" agent configuration"

Note: we show a preview of the data sources for the selected agent configuration. We also use this during the "enroll new agents" flyout #61076. We may want this to be a reusable component.

[elasticmachine]

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

[hbharding]

@nchaulet small update: we're going to hold off on building bulk actions in Fleet for Alpha 1. For now, users can access these actions on a per agent basis, by clicking the icon in the "actions" column, or on the agent detail page at the top where we'll have an "actions" button.