signedrequests: Pipe context in signing requests

[brancz]

Without this, our traces are often disconnected for requests that use request signing.

[alwaysmeticulous]

✅ Meticulous spotted 0 visual differences across 288 screens tested: view results.

Meticulous evaluated ~4 hours of user flows against your PR.

_{Expected differences? Click here. Last updated for commit eefadbf signedrequests: Pipe context in signing requests. This comment will update as new commits are pushed.}