SearchRule and SearchRuleAsync method contain bad naming

[nunomaduro]

Description

The following methods should be in plural: SearchRule => SearchRules and SearchRuleAsync => SearchRulesAsync to respect the API Client specs.

https://github.com/algolia/algoliasearch-client-csharp/blob/master/src/Algolia.Search/Clients/SearchIndex.cs:

        SearchResponse<Rule> SearchRule(RuleQuery query = null, RequestOptions requestOptions = null);

        Task<SearchResponse<Rule>> SearchRuleAsync(RuleQuery query = null,
            RequestOptions requestOptions = null, CancellationToken ct = default);

Goal: Deprecate the current methods with bad naming, and proxy them to new methods with the good naming.