chore: convert pipeline images from shell scripts to Go binaries

[richcooper95]

We have a couple of binaries that we run in Pipelines: reader and update-status. These are currently just shell scripts, but because of this, they're not very testable.

Currently, they're only covered in our system tests, which can't/shouldn't cover every edge case, so some testing is missing (and issues like #118 can occur).

This issue is to rewrite the scripts in Go, and add some unit tests, to ensure test coverage of these scripts.

(Note: There may be more scripts that we could convert too - reader and update-status are the two which seem most important, but if you find others, either raise a new issue or convert them as part of this issue depending on how big they are!)

[SaphMB]

A relevant bug: https://www.notion.so/syntasso/update-status-debugging-227a9404c114405c8129e8a0ab264834?pvs=4

[kirederik]

another bug on the shell script that could be more easily be fixed/picked up by a golang binary #138