faraday: Add Connection#build_url method signature

[ssshhhooota]

Add type definition for Faraday::Connection#build_url method.

The Faraday::Connection#build_url method builds a complete URI by combining the connection's URL prefix with a relative path and optional query parameters.

References:

https://github.com/lostisland/faraday/blob/c6988a840738760fae1a40d653fa2ccd0da425b9/lib/faraday/connection.rb#L415
https://www.rubydoc.info/gems/faraday/Faraday/Connection#build_url-instance_method

This is my first PR to this repository, so I'd appreciate a review if possible.

[github-actions]

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