Support and send Client Reports

[lcian]

These are useful to understand what's going on when events get dropped by the SDK.
See the develop docs

For logs, note the following, from @dingsdax:

• SDK must count each dropped log.
• SDK must calculate the size of each dropped log.
• SDK must emit both values as client outcomes.
• precise byte counting is not required; an approximate size is sufficient because:
  • Dropped logs are not billable.
  • sufficient to provide a rough overview to users (count + approximate size)

[lcian]

Apparently client reports are only supported in Sentry version v21.9.0 and later, we should keep that in mind and add it to the README for self-hosted users when we'll implement them.

[linear]

RUST-98