add sendPayloadChecksums config option and implement Bugsnag-Integrity header

[djskinner]

Goal

Add Bugsnag-Integrity request header (where required APIs are available)

Design

Changeset

• Set Bugsnag-Integrity in delivery-fetch, which is used in @bugsnag/web-worker.

Testing

• unit tests
• manually tested using the web-worker example: event appears when integrity header is correct, event does not appear in the dashboard when the integrity header is wrong
• no e2e tests as these APIs require a secure context (i.e. HTTPs) and our e2e tests don't support that yet