[ENHANCEMENT] add legend schema zod object

[shahrokni]

Relates to perses/perses#3014

Description

The idea is to create schema zod objects which will be used in plugins for Setting validations. Instead of creating a huge PR, the idea should be validated by submitting a small PR. So here, I have added the zod object schema for legend.
Eventually every schema under cue.common should have an equivalent zod object schema

Checklist

• Pull request has a descriptive title and context useful to a reviewer.
• Pull request title follows the [<catalog_entry>] <commit message> naming convention using one of the
  following catalog_entry values: FEATURE, ENHANCEMENT, BUGFIX, BREAKINGCHANGE, DOC,IGNORE.
• All commits have DCO signoffs.

UI Changes

• Changes that impact the UI include screenshots and/or screencasts of the relevant changes.
• Code follows the UI guidelines.
• E2E tests are stable and unlikely to be flaky.
  See e2e docs for more details. Common issues include:
  • Is the data inconsistent? You need to mock API requests.
  • Does the time change? You need to use consistent time values or mock time utilities.
  • Does it have loading states? You need to wait for loading to complete.