Overwrite generated Kubernetes pod specifications - BETA
## Description In this [MR](https://gitlab.com/gitlab-org/gitlab-runner/-/merge_requests/3114) we are releasing in [Alpha](https://docs.gitlab.com/ee/policy/alpha-beta-support.html?_gl=1*1iprpr8*_ga*NzYyODE4NDY3LjE2NjU3MTU3MDA.*_ga_ENFH3X7M5Y*MTY3Nzc5MjIxOC42NTEuMS4xNjc3Nzk0NTExLjAuMC4w#alpha-features) the ability for users to pass a [PodSpec](https://v1-18.docs.kubernetes.io/docs/reference/generated/kubernetes-api/v1.18/#podspec-v1-core) in either `JSON` or `YAML` format inside the `config.toml` of GitLab Runner. ## Proposal - Update the feature as needed based on user and customer feedback. - Release the PodSpec feature in [beta](https://docs.gitlab.com/ee/policy/alpha-beta-support.html?_gl=1*1iprpr8*_ga*NzYyODE4NDY3LjE2NjU3MTU3MDA.*_ga_ENFH3X7M5Y*MTY3Nzc5MjIxOC42NTEuMS4xNjc3Nzk0NTExLjAuMC4w#beta-features). ## How to use the Pod Spec Patch Feature in Runner {placeholder} ## Tasks to transition the feature from [`experimental`](https://docs.gitlab.com/ee/policy/experiment-beta-support.html#experiment) to [`Beta`](https://docs.gitlab.com/ee/policy/experiment-beta-support.html#beta) - [ ] Update doc with additional callouts - [ ] Create issues for refining the PodSpec feature for a GA release - [ ] Enable the `FF_USE_ADVANCED_POD_SPEC_CONFIGURATION` feature flag. Enabling the feature flag removes *a lot* of the configuration in the `config.toml`. [comment](https://gitlab.com/gitlab-org/gitlab-runner/-/issues/29659#note_1329042274) <!-- Please paste a link of the related issues or/and merge requests -->
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