feat: add possibility to save and delete prompts

[VinShurik]

Applicable issues

• fixes Add Prompt Save/Delete Support to Python Client #98

Description of changes

• Added save() and delete() methods for prompts in both sync (Prompts) and async (AsyncPrompts) clients.
• Implemented ETag-aware conditional operations (If-Match, If-None-Match) and mapped 412 responses to EtagMismatchError (keeping 404 -> ResourceNotFoundError).
• Added/expanded unit tests for save/delete success, header/payload behavior, URL validation, and error mapping.
• Added sync/async integration tests for prompt save/get/delete flows and ETag scenarios.
• Updated README Prompts section with new save/delete usage examples for both sync and async clients.

Checklist

• Title of the pull request follows Conventional Commits specification

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.