Add parsers for dashboard yml custom variables

What does this MR do?

This MR adds parsers to process templating variables that are passed as a part of the dashboard.yml file.

For iteration 1, only simple custom, advanced custom and text variables are supported. This MR, however, adds parsers only for simple custom and advanced custom variables.

Note: Parsers in this MR are not used in the monitoring dashboard yet. It will be in the next iterations

Simple Custom Variable

templating:
  variables:
    variable1: ['value1', 'value2', 'value3']

Advanced Custom Variable

templating:
  variables:
    variable1:                         # The name that can be used in queries in the form `%{variable1}`.
      label: 'Variable 1'              # The label that will appear in the UI for this dropdown.
      type: custom                     # We can add more types as we implement them.
      options:
        values:
        - value: 'value1'                # The value that will replace `${variable1}` in queries.
          text: 'Var 1 Option 1'         # The text that will appear in the UI dropdown.
        - value: 'value2'
          text: 'Var 1 Option 2'
          default: true  

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
• 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 #214513 (closed) #214533 (closed)