feat: make HTTP ref resolving be able to switched off

[pyrooka]

PR summary

This PR adds a new configuration option (through config file or CLI option)
which allows the users to disable the resolving of HTTP references during
the document processing.
Furthermore, a new section has been added to the README which clarifies
what kind of consumption of the project is supported.

PR Checklist

General checklist

Please make sure that your PR fulfills the following requirements:

• The commit message follows the Angular Commit Message Guidelines.
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• Dependencies have been updated as needed
• .secrets.baseline has been updated as needed
• npm run update-utilities has been run if any files in packages/utilities/src have been updated

Checklist for adding a new validation rule:

• Added new validation rule definition (packages/ruleset/src/rules/*.js, index.js)
• If necessary, added new validation rule implementation (packages/ruleset/src/functions/*.js, updated index.js)
• Added new rule to default configuration (packages/ruleset/src/ibm-oas.js)
• Added tests for new rule (packages/ruleset/test/*.test.js)
• Added docs for new rule (docs/ibm-cloud-rules.md)
• Added scoring rubric entry for new rule (packages/validator/src/scoring-tool/rubric.js)

[Andris28]

LGTM!