Add full-text search support

[quinnjr]

Description

Native full-text search support for PostgreSQL, MySQL, and SQLite.

Tasks

• PostgreSQL tsvector / tsquery
• MySQL FULLTEXT indexes
• SQLite FTS5
• Schema attribute @fulltext
• Query builder .search() method

Example

let posts = client.post()
    .find_many()
    .search("rust orm tutorial")
    .exec().await?;