Feature: add httpx transport config to client

[yanyongyu]

resolve #245

[Copilot]

Pull Request Overview

This PR adds support for custom HTTPX transport configuration to the GitHub client, enabling users to inject custom transport implementations for both sync and async operations. This is particularly useful for unit testing scenarios where mock transports can stub API responses without making real HTTP calls.

• Added transport and async_transport parameters to the GitHub client configuration
• Updated the client creation logic to use custom transports when provided
• Added comprehensive documentation and test examples demonstrating mock transport usage

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
githubkit/config.py	Added transport fields to Config dataclass and get_config function
githubkit/core.py	Updated GitHubCore init overloads and client creation methods to support custom transports
githubkit/github.py	Added transport parameters to GitHub class init overloads
tests/test_unit_test/test_mock_transport.py	New test file demonstrating sync and async mock transport usage
docs/usage/unit-test.md	Added documentation section explaining how to use test transports for mocking
docs/usage/configuration.md	Added documentation for the new transport configuration options

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