Replace openapi3 library with our own implementation

[vblagoje]

Description:
We are currently using an open-source OpenAPI invocation library that appears to be in maintenance mode, with a growing backlog of unmerged pull requests and unresolved bugs. These issues have caused significant challenges, particularly related to the inflexible response validation mechanism and various schema-related bugs. The current library's limitations hinder our ability to control and fix bugs in the API interaction layer, which is critical for our operations, especially as these do not align with the requirements of LLMs where schema validation is not a priority.

Proposed Solution:
Replace the existing OpenAPI library with our custom code which offers better error handling, eliminates outdated authentication mechanisms, and removes unnecessary JSON parsing processes. This change aims to provide a more robust, flexible, and maintainable framework for API interactions within our services.

Expected Benefits:

• Increased control and flexibility in handling API interactions and validations, allowing for optional response schema validation.
• Improved error handling with structured JSON error responses, enhancing the robustness of our service connectors.
• Simplified codebase by removing obsolete authentication mechanisms and redundant parsing logic, making the system easier to maintain and extend.

Impact:
This change will require updates to existing unit tests and the addition of a new integration test. It is crucial to verify that the transition retains all intended functionalities and integrates seamlessly with existing systems.

[julian-risch]

related PR: #7514

[vblagoje]

Completed and integrated in haystack-experimental with deepset-ai/haystack-experimental#8