This commit:
- Replaces `createRequest`, `createPost`, and `submitForm` with a single `sendRequest` helper.
- Adds `getRequest` and `postRequest` wrappers for clarity.
- Removes redundant content type constants and parameters.
- Updates envelope-related methods (`signEnvelope`, `rejectEnvelope`, `shareEnvelope`) to use the new request helpers.
- Simplifies `redirect` and `redirRejected` functions.
- Removed unused Content class
- Converted async arrow functions to standard function declarations
- Added default 'application/json' for createRequest and createPost
- Improved readability and consistency across API helpers
