Backend: Search Application - Added the post search functionality

[oguzhantok]

I have added the post search functionality, from now on users can search posts with the below service. There are two input parameters. query parameter is used to get input from the user and generate a search results according to given query. count parameter was developed to limit the number of posts in the response returned from the API.

Developed endpoint: /api/search/post?query={input}&count={input}

Other than that, Users search functionality improved. I have added the error conditions, number of users on the response and id values of the users.

[BeratDamar]

As a member of the frontend team, I will start to implement backend connection of the search bar for posts after @MercerFrey reviews your implementation.

Thanks for your contribution @oguzhantok