External Search SPA Integration

[g-saracca]

What this PR does / why we need it:

Implements the External Search functionality recently added in the backend side.
If there is more than one search service(default is Solr) then a Search Services dropdown is shown in the homepage search bar, as below 👇.

Which issue(s) this PR closes:

• Closes Implement External Search from Homepage #710

Special notes for your reviewer:

Tests are failing, those fixes are done in the release branch, will update this branch with develop once release is merged into it.

Suggestions on how to test this:

This can be tested in the QA environment, as an external search service has been configured in that environment. You can see from this Get search services endpoint result in the QA environment that there is another search service configured there.

The steps could be:

1. Go the homepage and select the "Natural Language Search" option. "Dataverse Standard Search" is Solr, should work the same way.
2. Enter a value in the search bar and submit the search. You will be redirected to the root collection.
3. Check the devTools => Network Tab => Filter by "search?", that is the search api request. The payload should have a 'search_service' property with the value of 'postExternalSearch'.
4. Subsequent searchs made from the collection page should use standard search.

That should be the only thing to test from the SPA, the obtained results depend on the external search service.

Does this PR introduce a user interface change? If mockups are available, please link/include them here: Yes

Is there a release notes update needed for this change?:

N/A

Additional documentation:

N/A

[ekraffmiller]

It looks good! Just a few questions.

[ofahimIQSS]

Hello! Please resolve the branch conflicts - thanks!

[g-saracca]

@ekraffmiller could you review again? I solved merge conflicts and made an update to avoid using the recently added picsum external source image with a fixture image. Same changes were applied in Advanced Search UI PR, if you could review that also will be great, thanks!

[g-saracca]

@ofahimIQSS, done here also

[ofahimIQSS]

tests passing - merging