feat: add fetch_api_boundary_nodes API to ic-agent

[nikolay-komarevskiy]

Description

API boundary nodes were recently added to the certified state tree. Currently, there are no API boundary nodes and the subtree is empty. accordingly. API boundary nodes will be added to the registry via NNS proposals at some later stage.

IC clients should be able to retrieve all these API BNs in a convenient way.

This PR extends ic-agent API with a new method:
pub async fn fetch_api_boundary_nodes() -> Vec<ApiBoundaryNode>

How Has This Been Tested?

1. Run this api_boundary_node_decentralization_test system-test. This test converts two unassigned nodes into API BNs.
2. Instantiate an agent with any IC replica url provided in system-test logs during test execution.
3. Retrieve API BNs via let api_bns = agent.fetch_api_boundary_nodes(effective_canister_id).await.
4. Verify that two API BNs are discovered within the State Tree.

Checklist:

• The title of this PR complies with Conventional Commits.
• I have edited the CHANGELOG accordingly.
• I have made corresponding changes to the documentation.