feat: adding support for request validation

[maticardenas]

issue: snok#291

Adding requestBody validation support.

Currently, the package offers the possibility of validating only the response, however, it is useful to also at the same time,e to have validated that the passed resquestBody in our functional tests are also properly documented and match the implementation.

Additions/Changes

• Methods for validating request (validate_request) and extract the corresponding requestBody schema section from the OpenAPI schema (get_request_body_schema_section)
• Updating test_project to also expose Pets API, as it is the OpenAPI schema we have in tests.
• Adding tests for schema tester, clients and validators.
• Updating affected variables and functions.

Notes

• Adding support only for openapi schemas (no swagger, previous versions), which involves StaticSchemaLoader and UrlStaticSchemaLoader instantiated with an openapi 3.x.x. This because it's the current requirement I have for my project.
• Decided to make request validation only when the scenarios has a successful response. Why? In the case you provide for example an incomplete payload, it will always fail naturally when comparing when your schema - in this situations we should let the functional case validate that the test we chose doesn't fit our design.

Follow Ups

• Add support for request bodies for swagger, DrfSpectacularSchemaLoader and DrfYasgSchemaLoader.
• Add support maybe for checking more aspects of the request? query params, headers, authentication, etc.