Store scheduling type of builds

What does this MR do?

This is the first MR of implementing empty needs for jobs in DAG pipelines

discussion: !21306 (comment 261625664)

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This MR will add scheduling_type column to ci_builds table to store scheduling type of builds.

It can be either stage or dag for now.

This will help us:

1. to understand how a build is scheduled. Ref: https://gitlab.slack.com/archives/C0SFP840G/p1576506635010000 (internal only)
2. to allow empty needs configuration for pipelines. (MR for this: !22246 (merged))

ref: #30631 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team