fix: allow user to customize certificate issuer group in case of existingIssuer is enabled

[maintux-nn]

Fixes #1798

This pull request enhances the configuration options for specifying an existing cert-manager issuer in the Metrics Server Helm chart. The main improvement is the addition of a new parameter to allow users to set the issuer's group, which increases flexibility for advanced cert-manager setups (e.g. aws-privateca-issuer which uses group awspca.cert-manager.io).

Enhancements to cert-manager issuer configuration:

• Added a new group field under tls.certManager.existingIssuer in values.yaml to allow specifying the group of the existing cert-manager issuer.
• Updated the documentation in README.md to describe the new tls.certManager.existingIssuer.group parameter and its usage.
• Modified the certificate.yaml template to use the new group value when configuring the issuer, ensuring the correct group is set for cert-manager resources.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: maintux-nn / name: Massimo Maino (0c298d5)

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[stevehipwell]

Thanks for the PR @maintux-nn, could you please add a CHANGELOG entry.

[stevehipwell]

/ok-to-test

[maintux-nn]

Thanks for the PR @maintux-nn, could you please add a CHANGELOG entry.

Hi @stevehipwell, CHANGELOG updated. Thanks

[dgrisonnet]

/assign @stevehipwell
/triage accepted

[maintux-nn]

Hello @stevehipwell, any news?