SOLR-17658: Implement start screen

[malliaridis]

https://issues.apache.org/jira/browse/SOLR-17658

Description

The start screen is the entry point where a connection to a Solr instance is established.

Solution

The implementation adds a new start screen where the user can insert a Solr URL (or use the default) and connect to a Solr instance. In case authentication is enabled for that specific Solr instance, it properly redirects to an authentication component (dummy component).

Tests

The PR adds a bucnh of tests that test both the new components and existing ones.

Checklist

Please review the following and check all that apply:

• [x] I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• [x] I have created a Jira issue and added the issue ID to my pull request title.
• [x] I have given Solr maintainers access to contribute to my PR branch. (optional but recommended, not available for branches on forks living under an organisation)
• [x] I have developed this patch against the main branch.
• [ ] I have run ./gradlew check.
• [x] I have added tests for my changes.
• [x] I have added documentation for the Reference Guide

[malliaridis]

@epugh Even though this PR is not ready to be merged yet, but maybe you still want to take a look?

[epugh]

I gave it a quick spin and it worked! Gave a reasonable error message back with no solr running, and then bin/solr start and it logged in. I didn't try it with a authenticated Solr however...

I also tried it with a old 8.11 Solr at http://quepid-solr.dev.o19s.com:8985 and it worked! The resulting pages didn't render the environment settings which I would assume is because of the age of the solr!

I suppose the follow on to this would be a logout button ;-)

[malliaridis]

I didn't try it with a authenticated Solr however...

This is actually the next step together with the logout button. :)

The resulting pages didn't render the environment settings which I would assume is because of the age of the solr!

Well, at first sight I thought this is expected behavior, but it was actually a bug. I forgot to update the URL in the HTTP client, so the request was sent to localhost instead. I fixed it and now it should work. As long as the API endpoints exist in the older versions, the UI should be able to render it correctly. We will add error handlings so that compatibility to older versions may be preserved to a certain degree. But one step at a time.

[epugh]

It worked with http://quepid-solr.dev.o19s.com:8987!