appsec: early blocking response

[eliottness]

What does this PR do?

• New Block() method in httptrace.responseWriter
• Remove dangling generator in contrib/net/http
• Rework blocking actions to use the Block() response writer method
• Move the call to the blocking handler directly in the OnData for HTTPAction method instead of calling it the end of the user request handler

Motivation

This PR fixes issues we (and users) can encoutered in two cases:

• if a blocking decision happens too late and the the response writer was already written to. Previously it would create a malformed request, now it does nothing.
• If after a blocking decision was taken, and for some reason the user code tries to continue writing on the writer;

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

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