Allow the URL of the shard to be overwritten, with multi shard url compatibility

Problem/Motivation

As seen in the parent issue, there is an incompatibility when the solr server defines its shards with replacement variables. This has been fixed, allowing the shard URL to be overwritten by the server URL of the Drupal Server entity.

Steps to reproduce

Same as parent. Set up a shard using a ${url.instance.url} style substitution variable (defined by the user manually) and clear caches. At this point the fields cannot be retrieved.

Proposed resolution

Replace the Boolean field with a field that allows you to define which URL should go for each of the configured shards.

Remaining tasks

Develop the logic to allow for the above

User interface changes

Changes in the Index configuration form

API changes

N/A

Data model changes

N/A