Add image source to telemetry data

[ciarams87]

Proposed changes

Problem: As a maintainer of NGF
I want to collect the image source from all NGF/NIC installations
So that I can understand the most popular methods for installing NGF/NIC

Solution: Export the BUILD_AGENT ARG in the container as an env var and use this value to determine if the image was:

1. built using the pipeline (i.e. prebuilt by us and pulled by the customer)
2. built locally using the make command or
3. of unknown origin (env var is not set to either "gha" or "local"

Testing: Unit tests and checked the logs for the value:

{"level":"debug","ts":"2024-02-21T11:27:35Z","logger":"telemetryExporter","msg":"Exporting telemetry","data":{"ProjectMetadata":{"Name":"NGF","Version":"edge"},"ClusterID":"0046b6ee-d5aa-4d7a-b426-1f968272e149","NodeCount":1,"NGFResourceCounts":{"Gateways":1,"GatewayClasses":1,"HTTPRoutes":1,"Secrets":0,"Services":0,"Endpoints":0},"NGFReplicaCount":1,"ImageSource":"local"}}

Closes #1310

Checklist

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• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

None

[kate-osborn]

LGTM