Handle default external id in SelectInput component (#1637)

[jochenklar]

This PR adds the handling of default external_ids in SelectInput component.

Resolves: #1637 .

[MyPyDavid]

we have an issue with this search definition in a (collection) question with an Optionset Provider plugin and multiple stored values. When the page opens it will make a lot of GET requests with a search, but those are not needed because a value is already there. So could we change this to:

const search = searchText

only?

PS, some infos by Codex:

getValueOption() already creates a temporary option from the stored value.external_id and value.text, so an existing value can be displayed without loading it again. The new useEffect in this PR would still explicitly load defaults that have an external_id but no text. Typed user searches would continue to work normally.
value.text is the persisted display label returned by the provider when the value was selected. Its main purpose here appears to have been reconstructing the selected option, but getValueOption() already handles that case.

[MyPyDavid]

It works, for example with the ROR Provider, when I set 05n911h24 as the Default external id but not when I set it to https://ror.org/05n911h24.
That seems a little counter-intuitive to me, but it is manageable by the editors. Only the relation between the "Default external id" and option set (the Provider) of the question may not seem so obvious.

[MyPyDavid]

No, wait. Adding a value to a collection question seemingly replaces the first entry of the default value instead of adding another entry to the collection. 🙄

[MyPyDavid]

thanks for including part of the fixes!
Now, when I have the ROR provider in a collection question with a default, then it will add the default ROR-id every time when I add "+Entry" another entry.
For the rest it seems good to me!