support for params having multiple values

[badnikhil]

PR Description

The request parameters were previously handled as a Map<String, String>. I have updated this to Map<String, List<String>> (string,dyamic at many places) to support multiple values when needed. Additionally, I have modified the relevant implementations to accommodate this change.

Related Issues

• Closes Can not pass multiple query parameters with same key name #268

Checklist

• I have reviewed the contributing guide.
• My branch is up to date and synced with the project's main branch before creating this PR.
• I am using the latest stable Flutter version (flutter upgrade verified).
• [] I have run all tests (flutter test), and they pass successfully.

Added/Updated Tests?

We encourage contributors to add relevant test cases.

• Yes
• No, because I plan to add tests after updating codegen for all supported languages.

Development & Testing Environment

• Windows
• Android
• Linux

[badnikhil]

@ashitaprasad I am familiar with the codebase; however, I need to explore the test files further before making the necessary updates. I plan to work on that after updating codegen for all languages. In the meantime, please review the implementation and share your feedback.