There should be a way to enable the extension at runtime

**Is your feature request related to a problem? Please describe.**
Yes - https://github.com/elastic/apm-agent-php/issues/627

**Describe the solution you'd like**
I would like to be able to set `elastic_apm.enabled = false` in the INI config, and then during runtime, enable it with a global `elastic_apm_enable()` type function.

**Describe alternatives you've considered**
I've tried using all the built in public API methods - none resolve the issue I mentioned above.

**Additional context**
I'm running a backend PHP daemon that forks itself to run worker jobs. I'd like to disable APM for the main daemon, and only enable it for individual jobs.

Currently my workaround is to use `dl()` on each job but that is a deprecated method with a bad performance impact.