Allow [runners.kubernetes.volumes] configuration in the helm chart

Description

A user can specify volumes when using the kubernetes execture. However when using the offical helm chart there is no possible option to set this up inside of values.yml

Proposal

When we reviewed https://gitlab.com/charts/gitlab-runner/merge_requests/39 we found certain issues and came up with the following proposal:

• Allow user to specify config template !1263 (merged), this is required because right now all configuration was done through environment variables and things like volumes are not supported by environment variable since they are a complex structure.
• Start using config template for configuration generation https://gitlab.com/charts/gitlab-runner/merge_requests/153
• Update https://gitlab.com/charts/gitlab-runner/merge_requests/39 to use the configuration template instead.

Links to related issues and merge requests / references

Community contribution MR : https://gitlab.com/charts/gitlab-runner/merge_requests/39