Query uses LIKE instead of = operator, add index on api_key

Problem

The api_key field is defined as a string field without 'case_sensitive' => TRUE setting. This causes Drupal's Entity Query to use LIKE operator instead of = for exact matches in the getUserByKey() method.

This results in:
- Slow query performance (700+ ms on very large user tables)
- Inability to use database indexes effectively
- Unnecessary case-insensitive matching for an exact-match field

Steps to reproduce

1. Install key_auth module
2. Create a user with API key
3. Monitor MySQL queries during authentication:

-- Current behavior (slow):
WHERE users_field_data.api_key LIKE 'xxx' ESCAPE '\\'

-- Expected behavior (fast):
WHERE users_field_data.api_key = 'xxx'

Here is an Entity SQL Condition class where we have LIKE instead of '=' when we don't have case_sensetive option enabled - https://git.drupalcode.org/project/drupal/-/blob/11.x/core/lib/Drupal/Co...

Proposed solution

Add case_sensitive => TRUE to the field definition in key_auth.module:

$fields['api_key'] = BaseFieldDefinition::create('string')
  ->setLabel(t('API key'))
  ->setDescription(t('The API key used for authentication.'))
  ->addConstraint('UniqueField')
  ->setSettings([
    'max_length' => 255,
    'text_processing' => 0,
    'case_sensitive' => TRUE,  // Add this line
  ]);

Also, add an update hook in key_auth.install to update existing installations.