feat(arcgis-rest-portal): add getFolderContent() to fetch user folder contents

[arcenter]

Resolves #769.

This PR adds the getFolderContent() function, allowing users to fetch content within a particular folder via a GET request to the ArcGIS REST API. This function also follows the standard paging patterns used across the API, such as num and start, allowing for consistent pagination of folder contents.

Changes

• Implements getFolderContent() in get.ts.
• Adds unit tests in get.test.ts to verify correct API calls and responses.
• Contains documentation with usage examples.

Testing

• Ran npm test to confirm all tests pass.

Note: Open to feedback, especially on implementation details.

[gavinr-maps]

@arcenter Thanks much for this PR. Once I started reviewing it, I realized that we already covered this endpoint in getUserContent(): #769 (comment). Sorry for not realizing that sooner. We appreciate the effort on this PR!!