feat: [CHK-1256] update openapi according to pagopa guidelines

[infantesimone]

List of Changes

• enrichment of the info section with description and externalDocs;
• rename resource target with payment-authorizations;
• refactoring request and response for xpay

Motivation and Context

The goal of this PR is to update openapi according to pagopa guidelines.

How Has This Been Tested?

https://editor.swagger.io/?url=https://raw.githubusercontent.com/pagopa/pagopa-payment-transactions-gateway/CHK-1256-review-openapi/api-specs/api_spec_PGS.yaml

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have created or updated an API interface.
  • I have updated the OpenAPI spec
• I have created or updated a database table
  • I have updated both the Oracle and Postgres migrations
• My PR is cohesive and self-contained (rule of thumb: less than 250 changed lines)

Link to story

[ciuffagianluca]

LGTM!