chore(examples): fix extensionserver build

[maxbrunet]

What type of PR is this?

Chore

What this PR does / why we need it:

Fixes:

$ make build
mkdir -p bin
CGO_ENABLED=0 go build -o bin/extension-server ./cmd/extension-server
# github.com/exampleorg/envoygateway-extension/internal/extensionserver
internal/extensionserver/post_cluster_modify.go:56:48: pool.Spec.TargetPortNumber undefined (type "sigs.k8s.io/gateway-api-inference-extension/api/v1".InferencePoolSpec has no field or method TargetPortNumber)
internal/extensionserver/post_route_modify.go:65:48: pool.Spec.TargetPortNumber undefined (type "sigs.k8s.io/gateway-api-inference-extension/api/v1".InferencePoolSpec has no field or method TargetPortNumber)
internal/extensionserver/post_route_modify.go:95:50: inferencePool.Spec.TargetPortNumber undefined (type "sigs.k8s.io/gateway-api-inference-extension/api/v1".InferencePoolSpec has no field or method TargetPortNumber)
make: *** [Makefile:18: build] Error 1

And:

$ make docker-buildx
Makefile:14: warning: undefined variable 'GO_LDFLAGS'
docker buildx build -f tools/docker/extension-server/Dockerfile . -t envoyproxy/gateway-extension-server:latest --build-arg GO_LDFLAGS="" --load
[...]
#5 [2/2] COPY ./bin/extension-server /usr/local/bin/extension-server
#5 ERROR: failed to calculate checksum of ref ac1187db-1850-4397-a0f7-3b6e6ef293d6::9cgc996t9aj67kkk02n8ld2cr: "/bin/extension-server": not found
[...]

Really lacking some basic automated build/lint/test 😅 Actually #7387 seems to catch it

(I also find a little strange that the port is never used in the cluster configuration)

Which issue(s) this PR fixes:

Follow up to #7389

Release Notes: No

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.27%. Comparing base (b10f1a2) to head (30724a9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7398      +/-   ##
==========================================
- Coverage   72.33%   72.27%   -0.06%     
==========================================
  Files         231      231              
  Lines       33999    33999              
==========================================
- Hits        24592    24573      -19     
- Misses       7639     7654      +15     
- Partials     1768     1772       +4     

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[arkodg]

thanks for cleaning this up