docs: add _userBy query and userByName built-in interface for Built-in IdP

[k1LoW]

This pull request updates the documentation to introduce and explain new ways to fetch IdP users by name, in addition to the existing methods for fetching by ID. It adds the new _userBy GraphQL query, describes its usage, and documents the new userByName client method in both API reference tables and code examples.

GraphQL API documentation updates:

• Added the _userBy query to the list of available IdP subgraph operations, allowing fetching a user by either ID or name.
• Documented how to use the _userBy query, including example GraphQL queries for fetching by id or name and explaining argument requirements.

Client library documentation updates:

• Added the userByName(name) method to the client API documentation tables and code examples, showing how to retrieve a user by name [1] [2] [3].
• Provided a new usage example for the userByName method in the "Managing IdP Users" guide, demonstrating how to fetch and return user details by name.