Marker payload data has no runtime type checks

When markers are loaded (from JSON) into perf, they have no runtime type checks or conversions on their contents. So while flow believes that certain fields have certain types, they don't necessarily. For example a lot of time fields are Numbers (Milliseconds) but in the JSON data are actually strings, nothing converts them from strings to numbers when they're loaded.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-721)