Plugin request state

[xtreme-vikram-yadav]

What this PR does / why we need it: This PR passes a shared partial octant state to the request objects passed to the plugins as arguments.

Which issue(s) this PR fixes

• Fixes Allow plugins to get the current label filter and observe changes #1512 Initialize plugins with current Octant state #1513

Special notes for your reviewer:

Some Design choices:

• The partial state is stored and passed via context. I felt conflicted about this choice as it didn't feel right to pass this data through context rather than a function argument. The reason context was picked to avoid updating multiple function arguments and interface implementation (which doesn't provide any meaning for the interfaces) along the multiple hops it takes for the state to go from state managers to plugins (for example: content_manager to a go plugin takes a lot of hops).
  This way is a bit obscure but provides meaningful function params and data can be tracked with as much difficulty as a function argument would provide.

• clientId and shared plugin state is kept separate as clientId is used in other places and it doesn't update the existing protobufs.

• plugins get the state as OState property describing partial octant state.

[wwitzel3]

I think I've sat on this long enough to decide I don't like the name OState .. it's too vague and isn't immediately obvious what it is. I think ClientState maybe a better choice.

Also, do we want to start to deprecate ClientID .. and move it in to ClientState ? So plugin authors get a ClientState object and it includes ID, Filters, Namespace.

I also think the state should include the current selected context.

[xtreme-vikram-yadav]

Good catch. I totally missed including context. I had considered adding clientID to the state but I didn't add it as it is used in other places separately and it avoided removing clientID from protobuf for existing plugins which might be using it separately. But passing the reference to the state is still a valid solution. I'll update it :)

[GuessWhoSamFoo]

lets squash some of these commits