Add agent pool relationship update APIs

[TanyaSingh369-svg]

Description

This PR adds dedicated agent pool relationship update APIs to improve parity and support full relationship replacement behavior for workspaces and projects.

Changes included

• Added dedicated relationship update methods:
  - update_allowed_workspaces()
  - update_allowed_projects()
  - update_excluded_workspaces()
• Added new option models:
  - AgentPoolAllowedWorkspacesUpdateOptions
  - AgentPoolAllowedProjectsUpdateOptions
  - AgentPoolExcludedWorkspacesUpdateOptions
• Added support for clearing relationship arrays by passing an empty list.
  - This resolves the issue where the generic update() flow skipped empty relationships and could not clear existing values.
• Updated agent pool examples to demonstrate:
  - assigning/removing workspaces
  - updating allowed workspaces
  - clearing allowed workspaces
  - updating excluded workspaces
  - updating allowed projects
  - clearing allowed projects

Output

Unit tests

Example validation

Executed:

PYTHONPATH=. python examples/agent_pool.py

Validated:
- allowed workspace updates
- excluded workspace updates
- allowed project updates
- clearing relationship arrays using empty lists
- workspace/project relationship assignment flows
- agent token creation and cleanup

Example completed successfully against a real TFE organization.