Add text variable parser for monitoring dashboard

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, simple/advanced custom, simple/advanced text variables are supported.

Simple/Advanced custom parsers have been added here - !31423 (merged)

This MR, adds simple/advanced text parsers for custom variables.

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

Simple Text Variable

templating:
  variables:
    variable1: 'default value'     # `text` type variable with `default value` as its default.

Advanced Text Variable

templating:
  variables:
    variable3:
      label: 'Variable 3'              # The label that will appear in the UI for this dropdown.
      type: text                       # The UI will have a free text box for this type, where the user can enter any value.
      options:
        default_value: 'default'       # The default value for this variable.

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)