[helm]feat: add fullname override option for gha-runner-scale-set

[c4po]

Background

I want to introduce a feature in the gha-runner-scale-set Helm chart that allows users to override the gha-runner-scale-set.fullname from the values.yaml file. This feature is necessary to support the installation of two runner scale sets in the same namespace with the same scale set name but targeting different GitHub configuration URLs. This is particularly useful for us to migrate runner sets from GitHub Enterprise Server (GHES) to GitHub Enterprise Cloud (GHEC).

Changes

• Introduced a fullnameOverride field in values.yaml to allow users to specify a custom full name for the runner scale set.
• Updated _helpers.tpl to use the override value if provided, enabling deployment of multiple runner scale sets in the same namespace with different GitHub configuration URLs.
• This change supports have multiple runner scale sets in the same namespace with the same scale set name.