Allow flexibility in automatic instrumentation

**Is your feature request related to a problem? Please describe.**
Currently we have a single `INSTRUMENTED` config value that turns _all_ instrumentation on/off. I think it would be useful to have some more flexibility.

In my particular use case for example, I would like to customise the `requests` instrumentation in my project which requires me to disable the out-of-the-box instrumentation. The easiest way I've found to do this is:
- Set `INSTRUMENTED: false` in config
- Vendor in logic found in `elasticapm.instrumentation.register`, `elasticapm.instrumentation.control`, to customising the `_cls_register` set to include my own instrumentation for `requests`.
- (Additionally for flask) Subclass `elasticapm.contrib.flask.ElasticAPM`, overriding the `init_app` method so that I can register the `request_started`, `request_finished` signals.

Side note re: flask - I think this may be a bug, depending on your perspective. In other places in the project e.g. Django instrumentation, the `INSTRUMENTED` config value refers only to instrumentation logic found in `elasticapm.instrumentation` (`AbstractInstrumentedModule` subclasses). Whereas in the flask module, it additionally prevents registering of the above flask signals.

**Describe the solution you'd like**
A way to customise exactly what gets instrumented from config, opt-in and opt-out.

**Describe alternatives you've considered**

Possible implementations:
- Exclusion/inclusion override lists of modules to instrument, e.g.
```
Config:
instrument_exclude = ["requests", "celery"]
instrument_include = [("requests", my_custom_instrumentation_module_name)]
```
or
```
Config:
instrument_overrides = [
("requests", my_custom_instrumentation_module_name), # to replace
("celery", None), # to disable
]
```

- Config value that overrides `elasticapm.instrumentation.register._cls_register`. A "raw" solution. This however would require the client to specify the module names which are internal to `elasticapm` (probably less than ideal).

Does anyone have any thoughts on this? Please correct me if I have overlooked something that makes this easy in the current version. Thanks.