Expose client state as an interface from service

[xtreme-vikram-yadav]

What this PR does / why we need it: This PR introduces client state as an interface to make it easy for plugin authors writing tests.

Which issue(s) this PR fixes

• Fixes ClientState uses a private struct #2305

Special notes for your reviewer:

Some design decisions:

• client state is duplicated for plugins and octant. With duplication comes the benefits of not having custom JSON handlers and both copies can now be managed and injected separately within plugins and octant core which provides better control.
• Plugins have access control over the state as the duplicated state is closer to plugins.

[ftovaro]

So if a I understand correctly what you did here, you created some methods to access the attributes of the ClientState in order to allow plugins authors to access those attributes without exposing the ClientState itself, right?

[wwitzel3]

Tom looked at this and was still having trouble because Filter type is internal. I think we need to lift that up as well.

[xtreme-vikram-yadav]

Tom looked at this and was still having trouble because Filter type is internal. I think we need to lift that up as well.

Ah right. I'll fix it.