Use client-credentials tokens for rpcinterface-full-stack-tests

[paulius-valiunas]

Some of our full stack tests use browser-based login, some use client credentials. We could keep the possibility to choose any authorization client, but each individual test should not be coupled to a specific authentication flow. I propose we change the tests so that they all use the same authorization client, and by default it's @itwin/service-authorization. Using the same authorization client everywhere would make it easy to expose additional options for it later.