[snowplow-analytics] ✨ Add optional consent context

[igneel64]

Description

Add a new schema describing the AMP consent state and the values we can track.

These data can come from the usage of the capability for an integration with a CMP (Consent Management Platform). Based on the settings on the CMP and the user selections, the values in the fields shown in the schema are updated and are available for us to collect.

Fields will be described in the schema.

• consentState
• gdprApplies
• consentType
• consentOne

closes #17

Pending schema snowplow/iglu-central#1329

[matus-tomlein]

LGTM (not knowing much about AMP)!

Just ran the schema through igluctl lint and it complains about 2 strings not having maximum length limit:

1. Following string properties provide no clues about maximum length (add --skip-checks stringLength to omit this check):
 - /properties/additionalConsentString	
 - /properties/consentString

Might be worth adding a limit just to avoid some huge events getting into the warehouse. Sometimes we use 4096 as max length but not sure what fits here.

Also there are no required fields in the schema – maybe that's intentional. On the other hand gdprApplies and purposeOne don't allow for null in their types. I think we should either make them required or add the null to the types. Otherwise we end up with a weird situation that they either are not present in the payloads or have a value but not null. But you'll probably get better feedback about this when you make the PR on Iglu Central.

[colmsnowplow]

Assuming CONSENT_METADATA macro works as it looks like it works, LGTM!