Add new service to create the web ide terminal

Instead of using the current CreatePipelineService in this MR we create a new service for building the web ide terminal.

This MR is based on the following assumptions:

• There is a different configuration file for this task apart from .gitlab-ci.yml. This file will be .gitlab-webide.yml and will be located inside the .gitlab directory
• This config file will allow only a small set of the CI syntax. In particular:
  • No global definitions are allowed
  • Only the job terminal will be defined in the file
  • The attributes script, before_script, image, and variables will be allowed
• This service won't add the new pipeline as the head pipeline of an MR, but it will be part of the MR's pipelines.

Port to CE https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23514

What are the relevant issue numbers?

Refs https://gitlab.com/gitlab-org/gitlab-ee/issues/5426

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?