Added REST methods for task and trigger validation that accept JSON body

[rybandrei2014]

What changes are being made and why?

Kestra doesn't provide rest endpoints for validation of task and triggers in JSON format. Endpoint for task/trigger validation /validate/task accepts only YAML body. Following PR adds two new endpoints:

• /validate/task - endpoint for task validation, accepts JSON body
• /validate/trigger - endpoint for trigger validation, accepts JSON body