Use api-client in framework plugins

[ishantiw]

Expected behavior

There should be a standard tool or way to interact with functional tests in plugin.

Consistently use lisk-api-client in the source code and for tests plugin's functional tests.

Actual behavior

We are using _channel property on application directly in tests in plugins or sometimes we use api-client

Which version(s) does this affect? (Environment, OS, etc...)

5.0.1

[shuse2]

Superseded by #6612