Make greentea standalone

[Patater]

We'd like to make greentea standalone again, rather than part of the Mbed OS mono repo. Let's re-use the greentea-client repo, make a new branch that is up-to-date with the latest greentea from Mbed OS, and get the project into a good standalone shape.

• Use this greentea-client repo
• Update this repo with code from the latest version of mbed-os (Sync latest code from mbed-os #19) @LDong-Arm
• Add empty implementation of greentea_metrics (Add an empty implementation for greentea_metrics #20) @rwalton-arm
• Remove mbed_trace dependency (Add an empty implementation for greentea_metrics #20) @rwalton-arm
• Remove references to Mbed from code/comments (greentea-client cleanup #23) @rajkan01
• Refactor Greentea client folder structure according to modern CMake Add support for stdio, CMake and example #21 @LDong-Arm
• Remove yotta references and files (including from README) (greentea-client cleanup #23) @rajkan01
• Document how to consume greentea-client using CMake (README.md: Document CMake build support #24) @LDong-Arm
• Ensure no Mbed tests in greentea-client (leave those in Mbed OS) @rajkan01
• Set up PR testing for greentea-client (GitHub Actions) @rwalton-arm (ci: Implement github action to build example app #22)
• Replace Mbed serial with stdlib putc and puts (via default implementation of interface functions e.g. greentea_putc()) (Add support for stdio, CMake and example #21) @LDong-Arm
• Remove dependency on greentea metrics reporting fixture from greentea-client @rwalton-arm
• Fetch greentea-client using FetchContent from mbed-os repo (extern/) @Patater
• Export CMake targets so consumers can use find_package or add_subdirectory @rwalton-arm
• Integration test and auto-update greentea-client within mbed-os
• Refactor Mbed OS's tests to use standalone version of greentea-client (when using CLI 2) @Patater
• Document how to use standalone greentea-client API @LDong-Arm (Improve documentation #30)
• Update Mbed documentation on how to produce the equivalent of greentea metrics (or remove docs if deprecating) Moved to greentea-client "metrics" API is unused mbed-os#14790
• Set up code coverage @rwalton-arm Add unit tests and code coverage #32
• Add unit tests @rwalton-arm Add unit tests and code coverage #32
• Add static analysis @Patater lgtm: Add intial LGTM configuration file #33
• Update README to remove GREENTEA_CLIENT_STDIO option, and explain how to link with libraries instead (README: greentea-client now namespaced #35) @LDong-Arm
• Use greentea-client from Mbed OS using FetchContent @Patater Use standalone greentea-client mbed-os#14803
• Refactor mbed-os' existing greentea io so it can be used by greentea-client Standalone greentea IO not working mbed-os#14893

[LDong-Arm]

All tasks on the list are done. A separate issue ARMmbed/mbed-os#14790 exists for deprecated metrics from the in-tree greentea-client used by Mbed CLI 1.