Make oauth2-proxy image configurable via --oAuth2ProxyImage

[Krutskik]

What it does

Closes #490

The operator previously hardcoded the quay.io/oauth2-proxy/oauth2-proxy registry
in templateDeployment.yaml, allowing only the tag to be configured via
--oAuth2ProxyVersion. This prevented deploying Theia Cloud in air-gapped or
corporate networks where images must be pulled from a private registry
(e.g. Nexus, Artifactory).

This PR adds a new operator argument --oAuth2ProxyImage that overrides the full
image path (registry + repository) for the oauth2-proxy container, while
--oAuth2ProxyVersion continues to control the tag.

Changes

• TheiaCloudOperatorArguments: new --oAuth2ProxyImage option
  (default quay.io/oauth2-proxy/oauth2-proxy), plus getter and updated
  hashCode/equals/toString.
• DefaultDeploymentTemplateReplacements: new placeholder-oauth2-proxy-image
  placeholder, populated from the new argument in both getReplacements methods.
• templateDeployment.yaml: image line changed from
  quay.io/oauth2-proxy/oauth2-proxy:placeholder-oauth2-proxy-version
  to placeholder-oauth2-proxy-image:placeholder-oauth2-proxy-version.
• CHANGELOG.md: added entry under Unreleased.

Backwards compatibility

Fully backwards compatible. The default value reproduces the previous behavior,
so existing deployments are unaffected. Users in restricted networks can now set
e.g. --oAuth2ProxyImage=nexus.corp.local/oauth2-proxy/oauth2-proxy.

Follow-up

The Helm chart lives in the separate repository. A companion
change is needed there to expose this as a value (e.g. operator.oAuth2ProxyImage)
and pass it through to the operator. Happy to open that PR as well.

[lucas-koehler]

Hi @Krutskik , thanks for the contribution ❤️
This extension in configurability definitely makes sense to me 👍
Before we can accept any contributions, sign the Eclipse Foundation's contributor agreement, please.
A PR for the companion change on the Helm repository would be appreciated 😊

[Krutskik]

Hi @lucas-koehler !
I will sign agreement and will make PR to the Helm repo. I have a little problem with registration on the website, but I hope I'll figure it out soon

[Krutskik]

Hi @lucas-koehler ! I signed the agreement 😊

[lucas-koehler]

Hi @Krutskik , thanks for signing this and the Helm repo PR. I'll review both later this week

[lucas-koehler]

Hi @Krutskik,
the changes LGTM! Thanks for the contribution ❤️
I'll merge this together with the Helm change once it is approved, too :)