This draft instrumentation gateway allows to listen and react to specific
operation stacks. In this first attempt, it tries to avoid maps of operation
attributes and rather relies on Go structures. This allows to simplify the
event listener and emitter API by relying on the operation arguments/results
structure type. It also allows have strongly types lists of operation
arguments and results.

The main limitation comes from the absence of Go generics for now, that
implies the use of empty interface{} values along with some calls to the
reflection package.

Quick POC of the WAF using an operation event listener on the HTTP handler operation.
It passes some of its values to the WAF and runs it. For now, it panics when an error
or an attack occurs.

By enforcing the order of the WAF listener so that it properly comes after the data events,
it is then possible to only rely on them and have a cleaner callback.

This shows that a better abstraction should be developped in the future so that no such manual
hacks in the listener order is required.

Remove the use of reflect.Type as map key type to avoid possible panics if in
the future the `reflect` package returns a non-hashable type descriptor.

To uniquely identify the operation argument/result type as it was done with
reflect.Type, a struct is now used which includes the type package path and
name. This avoids run-time concatenation between reflect.(Type).PkgPath() and
reflect.(Type).Name() whereas the struct simply references those two strings.

Note that anonymous structs shouldn't be allowed as they have no name nor
package path.

- Instrumentation Gateway implemented by package dyngo
- WAF integrated behing dyngo's operation events
- Simple HTTP client and security event batching
- Separate AppSec Go module

Start/Finish the abstract HTTP handler operation using dyngo along with
the data required for the AppSec WAF and its security event context.

Integrate AppSec behing the APM tracer so that existing APM users
can transparently benefit from AppSec by simply updating the APM
module. It is disabled by default, unless DD_APPSEC_ENABLED is true.

Co-authored-by: Gabriel Aszalos <[email protected]>

Considering dd-trace-go as the overall Datadog's client library for Go, appsec can move to
internal/ and doesn't need to be a standalone module.

Another consequence is that dyngo can go into internal/, where it acutally belonged as it is
not an appsec-specific package.

Remove the context as it is already possible to stop the appsec goroutine without the context.

…/waf

Register() functions now return their unregister function closures to call in order
to unregister the instrumentations and their callbacks. dyngo.Unregister() was
removed.