3814 Path provider windows crash fix (cla: yes, p: path_provider, platform-windows)

3815 fix incorrect MD in CONTRIBUTING.md (cla: yes)

3816 [google_sign_in_web] fix README typos. (cla: yes, p: google_sign_in, platform-web)

3817 [url_launcher] Add a workaround for Uri encoding (cla: yes, p: url_launcher)

3818 [tools] fix version check command not working for new packages (cla: yes)

3819 Add a todo WRT correctly setting the X-Goog-AuthUser header (cla: yes, p: google_sign_in)

3821 [in_app_purchase] platform interface improvement (cla: yes, p: in_app_purchase)