Enable kill_request filter in the proxy build

[jwendell]

The kill_request extension has enabled_default = False in its Envoy BUILD file, so listing it in extensions_build_config.bzl alone is not enough — it compiles as an empty target. Add the Bazel flag to .bazelrc so the filter is actually included in the binary.

[jwendell]

The issue here is: We think we are building with this extension, since it's in the extensions_build_config.bzl file, but we need this flag in addition. This PR fixes this mistake. Alternatively we could simply remove that extension, but I think the original intention was to include it.

[zirain]

@kyessenov can you recall why it's disabled in the past?

[jwendell]

That extension was added in Envoy disabled by default: envoyproxy/envoy#14170 - A new mechanism to enable it was added after that: envoyproxy/envoy#14240

In istio/proxy this extension was always included in our configuration since day 0, but it's not built by default unless a build flag is specified.

This PR adds this build flag.

My intention is to avoid confusion when reading the extensions_build_config.bzl file. One might think this extension is enabled when in fact it isn't.

Given it was never enabled in Istio, I think the best approach then is just remove it from extensions_build_config.bzl.

WDYT?