feat: add context value flag

[alemrtv]

• add the --context-value command line flag to pass arbitrary key value pairs to the evaluation context

Fixes #1435

Follow-up Tasks

• adjust the integration test to include this flag

How to test

issue the flagd start command with the -X flag and provide key value pairs like:
flagd start -X name=jane -X age=31 or flagd start --context-value name=jane --context-value age=31

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[beeme1mr]

Hey @alemrtv, sorry for the delay on the review. I'll take a look as soon as possible.

[aepfli]

Thank you. Those changes look great from my perspective. I added a few remarks for ease of maintenance, which would also reduce the size of this PR, and the lines changed.

I don't have reviewer power here ;( but those are really good changes and improvements; thank you.

[toddbaert]

Hey @alemrtv ! Sorry for the delayed review... Many of us were away with Kubecon and still playing catch up. I'll review this next week, latest. 🙏

[toddbaert]

Manually tested and works as expected both for evaluations and GetSyncMetadata calls 🙌 - Amazing work!

I only have one small question for your consideration: #1448 (comment)

I will merge this by the end of the week unless I hear objections from other contributors.

[toddbaert]

I've pushed a small change to fix the lint/doc CI failures as well as adding a new make target.

[toddbaert]

Hey @alemrtv , thanks again. I pushed up one more small change which again changes the priority of the "configuration level" context attributes (we'll call them static attributes) vs incoming request based context attributes. I will explain why here for your understanding and for posterity:

Basically, we have 2 modes of operation for flagd providers (clients): in-process and rpc. In RPC mode these static attributes are injected into flagd's evaluation logic (this is what you've done), while in in-process mode, these values are delivered via the GetSyncMetadata call, and the providers on the other end of the wire inject these properties into their local evaluation. The way they do that is via a provider hook, which has the highest priority in terms of context merging. We can't change that priority rule, so for consistency, we've opted to also have the same priority in RPC mode. I hope that makes sense! I explained the same to @beeme1mr and @aepfli and they are on the same page. At the end of the day, I hope people generally avoid these sorts of collisions anyway.

Again, thanks for your help. I will merge this PR this week and follow up with a bunch of documentation about this feature (keeping it somewhat hidden for now since many providers don't yet support the injection I mentioned above).

Eventually, we should also enhance our gherkin suite to do some assertions which assume some of these static attributes are configured to verify consistent behavior.