[CHORE] Backfill ADR for static context and dynamic context enrichment

[cupofcat]

We need to document the behavior and purpose of static and dynamic context enrichment in the flagd daemon (basically the purpose and use-cases of the -X and -H options here).

Be sure to include:

• basic summary and use cases for -X option (static context enrichment), such as setting a cloud-provider or AZ for the flagd instance and all contexts in it
• basic summary and use cases for -H option (dynamic context enrichment), mapping HTTP header values to (such as a user-locale or browser) to contexts for that evaluation
• override priority for the above (highest priority > header-context-from-cli > static-context-from-cli > request-context > lowest priority)
• how -X options are delivered to in-process providers via the sync-context

[toddbaert]

@leakonvalinka you worked on some of this, might be a good one for you to document.

[leakonvalinka]

I could definitely give it a try (I'm away next week though, so it wouldn't be for another two weeks)