Fix parsing of Infobip whatsapp API schema

[sternakt]

Description

• Patch datamodel-code-generator issue
• Fix multiline descriptions parsing
• Add "patch" method to OpenAPI client
• Reroute whatsapp tutorial and tests to https://dev.infobip.com/openapi/products/whatsapp.json
• Remove postman tests from CI/CD

Fixes #511
Fixes #446

Type of change

• Documentation (typos, code examples, or any documentation updates)
• Bug fix (a non-breaking change that resolves an issue)
• This change requires a documentation update

Checklist

• I have conducted a self-review of my own code
• I have made the necessary changes to the documentation
• My changes do not generate any new warnings
• I have added tests to validate the effectiveness of my fix or the functionality of my new feature
• I have included code examples to illustrate the modifications

[codecov]

Codecov Report

Attention: Patch coverage is 83.67347% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ency/api/openapi/patch_datamodel_code_generator.py	83.33%	4 Missing and 4 partials ⚠️

Files with missing lines	Coverage Δ
docs/docs_src/tutorials/whatsapp/main.py	47.82% <100.00%> (ø)
fastagency/api/openapi/client.py	85.33% <ø> (ø)
...ency/api/openapi/patch_datamodel_code_generator.py	83.33% <83.33%> (ø)

... and 7 files with indirect coverage changes