[build] Add CircleCI iOS & macOS builds (master branch)

[friedbunny]

Cherry-picks #10257 into master. This replaces the Bitrise iOS and macOS builds with improved versions on CircleCI.

Note that this does not replace the macOS-based Bitrise Node and Qt builds — for that, see #10278.

Once this PR is merged, we should:

• Un-require the Bitrise iOS and Bitrise macOS builds on GitHub.
• Require the three new CircleCI builds (ios-debug, ios-sanitize-thread, and macos-debug) on GitHub.

After this is merged, if you find that your PR fails to start some required tests: rebase onto master to bring in these changes.

/cc @mapbox/gl-core @mapbox/android @mapbox/ios

[friedbunny]

The new builds are failing because #10190 isn’t in master yet:

- missing symbol export for class MGLMapSnapshotOptions in platform/darwin/src/MGLMapSnapshotter.h:12:12

@fabian-guerra @tobrun Have we thought about merging release-agua back to master (now that beta 2 is released), or should I just cherry-pick what I need?

[friedbunny]

Holding on this until after ios-v3.7.0-beta.3 is tagged and merged back to master.

[brunoabinader]

Holding on this until after ios-v3.7.0-beta.3 is tagged and merged back to master.

This is now done via #10421.

[1ec5]

#10257 itself was merged back to master, so this PR is no longer necessary per se. However, the GitHub hooks still need to be updated to require CircleCI instead of Bitrise.

[friedbunny]

• The three new CircleCI builds (ios-debug, ios-sanitize-thread, and macos-debug) are now required on the master branch.
• The iOS and macOS Bitrise builds are un-required on master and will always pass.
• There are two remaining builds on Bitrise (macOS-based node and Qt) — for the migration of those, see [build] Add CircleCI Qt & Node macOS builds #10278.