Pre-populate variable keys + value for manual pipelines

Problem to solve

Having to remember exactly the variable name and possible values when specifying variables for manual pipeline

Proposal

• Add a combo-box, with prefilled values pre-populated variables into pipelines

Further details

• Prepopulating the variable keys. I'm not sure how these could be defined, perhaps any top-level variables defined in .gitlab-ci.yml could have entries with their names already filled in. This way we dont have to remember exactly what the variable name is supposed to be.
• A dropdown list or "enum" of values in addition to just text input. This could be defined in the top-level variables as well though i'm not sure how you'd tackle setting its value for pipelines created by new commit pushes.

Links / references

https://gitlab.com/gitlab-org/gitlab-ce/issues/44059

Customer ticket -> https://gitlab.zendesk.com/agent/tickets/97958 (Internal)

https://gitlab.my.salesforce.com/0016100001ELkxW?srPos=0&srKp=001 (internal link)