Extract custom value stream form fields

What does this MR do?

This MR is some ~"feature::maintenance" to simplify the implementation of #267536 (closed). We will still need the original form while we migrate to the new layout, but will need to use the same fields in the new form, so this MR allows us to reuse the form fields without breaking the existing form.

• Extracts the form fields for custom value stream into a new component
• Moves some related functionality into utils and constants
• Replaces finders with data-testids

This MR will be followed up by:

• !50136 (merged) - adds feature specs for the custom stage fields component
• !49651 (merged) - refactor remaining custom stage form specs

Screenshots (strongly suggested)

No visual changes

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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

Related to #267536 (closed)