Add client side Segment integration, making it work with web analytics and session properties without customization

[robbie-c]

Feature request

Is your feature request related to a problem?

Right now, our Sentry integration is server side (cloud mode) and doesn't send a session id with the events. This means that session properties do not work, and neither does web analytics which makes heavy use of session properties.

Our existing approach is to tell people to send their own session Ids.

(You can read about why segment doesn't have sessions here)

Describe the solution you'd like

Another approach would be to create a web mode Segment destination, which could handle this automatically

Describe alternatives you've considered

Update the docs to make it easier for people to send their own session Ids. We'd want to update the segment docs too.

Additional context

We should update the docs for the cloud mode integration anyway