feat: accept other JSON types as distinct_id, stringify them

[xvello]

posthog-js accepts any JSON value as distinct_id and capture-py just stringifies them before truncating to 200 chars.

• Because being 100%-compatible with the map stingification that python does would be a pain, I didn't go into that rabbit-hole and used serde's stable one (keys are sorted, no extra whitespaces). This might churn persons for some
• capture-py accepts null as a value, but that feels far-fetched as the value is required for the event to be valid. I'll go with the breaking change in that PR, and check ClickHouse to see how many teams would be impacted.