Fix perform readonly transformation for lazyloaded searchabledropdown

[johannesx75]

Description

performReadonlyTransformation() on a SearchableDropdownField or SearchableMultiDropdownField, will convert the source DataList into an array, which requires pulling data for all records in the list from the database.

This is caused by the performReadonlyTransformation of the underlying SingleSelectField and MultiSelectField calling getSource. To circumvent this a new LazyLoadedLookupField is used. It has the same logic as SearchableDropdownTrait to use a Datalist internally for the Source.

This also contains the fix for #11294, since the wrong initial value of "Title" was messing up the control.

Please note: This solution feels like quite a hack. I really don't like the fact, that getSource is not really returning the source. But I figured as long as SearchableDropdownTrait works this way the performReadonlyTransformation should be similar to it.

Manual testing steps

Given ModelA has_one ModelB and ModelB has_many ModelA:

• Open ModelB in Model Admin
• Go to the ModelA Tab
• Click on Add new Model A
• A read only Field should be rendered and only one ModelB will be loaded from the Database

Issues

• performReadonlyTransformation() for SearchableDropdownField and SearchableMultiDropdownField doesn't respect lazy loading #11293
• DBForeignKey scaffolding uses wrong parameters to create SearchableDropdownField #11294

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[emteknetnz]

Have tested locally, works well, just a few small changes to make

Not that I've changed the base from 5.2 to 5 (so will be released in CMS 5.3), because this is adding new API so to follow semver it needs to be released in a new minor

You'll need to rebase on top of 5 as there's now a merge-conflict

[johannesx75]

@emteknetnz, sorry for the late response. I was out sick.

Think I've made all the requested changes. One of the checks fails: 'CI / CI / 8.1 mysql57 phplinting (pull_request)'. From what I can tell thats unrelated to my changes.