Make text property optional for custom var types

What does this MR do?

The monitoring dashboard supports custom-type templating variables https://docs.gitlab.com/ee/user/project/integrations/prometheus.html#full-syntax-1. The current implementation expects the option to have both text and value properties.

This MR makes the text optional and uses value as text in such cases

Templating variable formats supported

Before

templating:
  variables:
    variable1:                         
      label: 'Variable 1'              
      type: custom                     
      options:
        values:
        - value: 'value1'                
          text: 'Var 1 Option 1'         # <------------ This prop should be optional
        - value: 'value2'
          text: 'Var 1 Option 2'         # <------------ This prop should be optional
          default: true                  

After

templating:
  variables:
    variable1:                         
      label: 'Variable 1'              
      type: custom                     
      options:
        values:
        - value: 'value1'                
        - value: 'value2'
          default: true                  

Screenshots

Before	After

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

Mentions: #218450 (closed)