Validate references in validateDoc

[PARAIPAN9]

Motivation

• Fixes #341.

Modifications

• Develop a function responsible for iterating through and validating the references within the OpenAPI document. If external references are present or if they are not found in the 'components' section, the function should throw errors.

Result

• This will help catch invalid references before code generation.

Test Plan

• Make sure that tests pass and wrote additional unit tests.

[simonjbeaumont]

@swift-server-bot test this please

[simonjbeaumont]

Thanks @PARAIPAN9!

[PARAIPAN9]

You're welcome, always a pleasure to contribute!!