Improvements and bug fixes.

Added

- ``SAClient.list_users`` method lists contributors with optional custom field filtering.
- ``SAClient.get_user_metadata`` method retrieves contributor metadata, including option for custom fields.
- ``SAClient.set_user_custom_field`` method sets a custom field for a contributor.
- ``SAClient.pause_user_activity`` method pauses a contributor's activity on specified projects.
- ``SAClient.resume_user_activity`` method resumes a contributor's activity on specified projects.
- ``SAClient.list_projects`` method lists projects with optional custom field filtering.
- ``SAClient.set_project_custom_field`` method sets a custom field for a project.

Updated

- ``SAClient.upload_annotations`` added ability to specify the format for processing and transforming annotations before upload, including Multimodal projects.
- ``SAClient.get_annotations`` added ability to specify the format for processing and transforming annotations before retrieving them.
- ``SAClient.list_items`` added ability to retrieve item categories.
- ``SAClient.get_project_metadata`` added ability to retrieve custom fields of projects.