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

[netlify[bot]]

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[beeme1mr]

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

[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. 🙏