Use /multi-search for better performance

[BertrandBordage]

Currently, wagtail-meilisearch queries each meilisearch index one by one and gathers results:

wagtail-meilisearch/wagtail_meilisearch/backend.py

Lines 419 to 426 in 49ce381

	for m in models:
	index = self.backend.get_index_for_model(m)
	result = index.search(terms)
	boosts = self._get_field_boosts(m)
	for item in result['hits']:
	if item not in results:
	item['boosts'] = boosts
	results.append(item)

On a website with dozens of page types, searching with meilisearch on the global admin search can take more than a minute.

Would it be possible to use the /multi-search endpoint to send all these "subqueries" at once? I did a test using 3 indexes, the latency is the same as with 1 index, so we can probably expect much better performance with this approach.