docs(ADR): standardize Flag configuration parsing and error handling

[aepfli]

relates to: #1690

This is the starting point, i think we should talk about what we want to achieve and how. My simple mind did not manage to come to another option i could propose, but i am open to add more options. please let me know what you think of this.

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[toddbaert]

@aepfli sorry, I started typing without selecting the textbox first and accidently closed the issue :sweat_smile:

To summarize my alternative proposal:

• use https://flagd.dev/schema/v0/flags.json to validate the whole document, on each sync
• at startup, if anything is invalid, flagd fails to start (this is already partially implemented; if we can't parse the flags configuration, so we'd just follow that same path)
• on any sync update, if anything is invalid, the update is ignored and an error/warning message is logged louded (again, this is already partially implemented; if we can't parse the flags configuration, so we'd just follow that same path)

Advantages:

• very easy to implement, just add a validation step in the exact same code-location we already parse
• we can remove many of our "error-edge-cases" in our test suite, since they will never be allowed
• we rely fully on a schema for validation so we don't have to implement any detailed error handling code

Disadvantages:

• 1 bad flag will prevent a whole update - however, this is already true if a single flag cannot be parsed (invalid JSON or nonsense variant configuration, etc) so we can never fully remove this situation, so I think we should just embrace this flow

[aepfli]

@toddbaert I also added your proposal.

I think it would be time to start some kind of vote - or give people the chance to add their remark - @open-feature/flagd-triagers please take a look