Thread Safe mode (ZTS) support

**Is your feature request related to a problem? Please describe.**
Testing the agent with FrankenPHP server which runs PHP in ZTS mode yields the extensions unable to load:

```log
INFO [elastic_apm_loader][PID: 136][TID: 148] Thread Safe mode (ZTS) is not supported by Elastic APM Agent
```

**Describe the solution you'd like**
Adding support for Thread Safe mode (ZTS)

**Describe alternatives you've considered**
The only alternative I can think of is building a custom FrankenPHP version with APM agent extension compiled In, which is not viable due to:
- Requires complex multi-stage Docker build
- Significant build time
- High maintenance burden for updates
- Build complexity makes debugging difficult

Thanks for the awesome work your team is doing