Add cmake code for performance module

[gwaldvogel]

Description

As discussed in https://github.com/firebase/firebase-cpp-sdk/issues/671 this PR provides some basic CMake and gradle code to compile the performance module.

---

Testing

I have currently only tested the Android and iOS binaries within our internal project.

---

Type of Change

Place an x the applicable box:

• [ ] Bug fix. Add the issue # below if applicable.
• [ ] New feature. A non-breaking change which adds functionality.
• [X] Other, such as a build process or documentation change.

---

Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

[jonsimantov]

Thanks for sending this! Since Performance is not officially supported in the C++ SDK yet, I'd like to ensure that this is not included in builds by default, so could you change the default setting to reflect that? Developers who want to include it can specify -DFIREBASE_INCLUDE_PERFORMANCE=TRUE.

Also, could you ensure that the stub build compiles on at least one desktop platform (e.g. Mac)?