[Logs UI] Pin field as column

[makwarth]

Implementation issue for #29415

High-level task summary

• Introduce concept of field columns in the UI. Each column will display field value of respective log event.
• Add column configuration to the source configuration ([Logs UI] Add Columns to source configuration #31462)
• Add ad-hoc column configuration to the Customization popover. Customization configuration is transient. If the user wants to make the changes permenent, he/she will need to manually update the source configuration.
• NB: The message field column is special as it has logic to it: If there's no message field, it'll try to find a suitable log formatting rule and use that to generate the message.

Wireframes

Columns

Customization

• The first [icon] is a drag icon (horizontal bars). The user can drag and drop on this icon to reorder the column order. (This feature is not required for the first iteration.)
• The second [icon] is a trash icon. Clicking this icon will remove the field column.
• Customization contents have been reordered and made more compact

[elasticmachine]

Pinging @elastic/infrastructure-ui

[weltenwort]

With #34916 the log view now has column which can be configured via the source configuration. It was agreed that a non-persistent override for the column configuration is not required for now and that the following aspects be broken out into separate issues:

• Easy reordering of columns: [Logs UI] Make column configurations reorderable #35736
• Column headers: [Logs UI] Add column headers #35780
• Easy access to the column configuration: [Logs UI] Provide easy access to the log column configuration #36105