[REQ][Ruby] Add support for faraday 2.x

Created by: ykitamura-mdsol

Is your feature request related to a problem? Please describe.

Faraday 2.x is released: https://github.com/lostisland/faraday/discussions/1358

Describe the solution you'd like

Based on the UPGRADING guide, we need to make changes to the following files:

gemspec.mustache

Relax Faraday dependency and add s.add_runtime_dependency 'faraday-multipart'

https://github.com/OpenAPITools/openapi-generator/blob/510f79989a003d2c81e6960ef09e8ba3ee43843d/modules/openapi-generator/src/main/resources/ruby-client/gemspec.mustache#L22-L24

api_client.mustache

Add require 'faraday/multipart' if Gem::Version.new(Faraday::VERSION) >= Gem::Version.new('2.0')

https://github.com/OpenAPITools/openapi-generator/blob/510f79989a003d2c81e6960ef09e8ba3ee43843d/modules/openapi-generator/src/main/resources/ruby-client/api_client.mustache#L13-L15

api_client_faraday_partial.mustache

Handle basic_auth like this to support both Faraday 1.x and 2.x.

https://github.com/OpenAPITools/openapi-generator/blob/510f79989a003d2c81e6960ef09e8ba3ee43843d/modules/openapi-generator/src/main/resources/ruby-client/api_client_faraday_partial.mustache#L16

I'm going to work on this if no one has already started.