feat: allow hostname override

[nicklasfrahm]

What this PR does / why we need it:

Our API management uses Kong and we have dataplane deployments in multiple regions. At the moment our lovely uber-API guy REMOG creates a hardcopy of the helm chart, which drastically increases maintenance effort on his side.

This is the only feature he is missing, so the platform guy (me) is helping out.

Checklist

• [x] PR is based off the current tip of the main branch.
• [x] Changes are documented under the "Unreleased" header in CHANGELOG.md
• [x] New or modified sections of values.yaml are documented in the README.md
• [x] Commits follow the Kong commit message guidelines

[CLAassistant]

All committers have signed the CLA.

[remogvestas]

we use the hostname variable to have a saying hostname in the response of the /clustering/data-planes endpoint, like:

data: [ [-] { [-] config_hash: b4d10b38ea9c510b0d5590c2c04a4fdb hostname: dataplane-git-weu id: 010be8cf-c5f9-42ea-b94f-76a3a05bda2c ip: xx.xx.xx.xx last_seen: 1700132083 sync_status: normal ttl: 1209581 version: 3.1.1.1

  so we easily can check the status of all dataplanes across all regions and instances