Update api to include agent inject feature

[zippy]

The PR adds the agent injection that was added to the admin API in holochain/#501

[Connoropolous]

Just an initial scan...

[Connoropolous]

I really like that this is possible now because not only is it good for the test framework, but it will be good for other third party peer discovery solutions and options.

[Connoropolous]

@philipbeadle did the tests pass locally?
CI is failing with:

   can inject agents
    ✔  should be truthy
    ✔  should be truthy
    ✔  should be truthy
    ✖  [object Object] at undefined
        undefinedundefined
        Conductor exited with code null
        Conductor exited with code null

[freesig]

Kepp the agent info type opaque because it's not using holochaib types like AgentPubKey it's using kitsune type which you don't have here