Add hover for model associations

[vinistock]

In #358, definition support for associations was added. We can do the same thing for hover, which would be quite nice in my opinion.

Implementation

1. Start returning the fully qualified name of the constant related to the association from the server here
2. Generalize this API to return the full information about an association, rather than being specific about locations only
3. Fix the API changes in definition.rb
4. Then use a similar approach to produce hover content

[andyw8]

(implementation was in https://github.com/Shopify/ruby-lsp-rails/pull/373)