Add Additional Documentation to reflect codespaces configuration requirements

@timovv and co discovered a new conflict when attempting to `test-proxy push`. Specifically, the github PAT that is created and set within the codespaces has minimal permissions, and doesn't have `write` access to the `assets` repo even if their _user_ may.

copy/pasta from @timovv 's fix to FIX this behavior for codespaces created off `main` in the `azure-sdk-for-js` repo.

> GitHub grants minimal permissions to a Codespace when it is created through creating a personal access token (PAT). By default, this PAT only grants write access to the repo that the Codespace was created from. This causes permissions issues when pushing assets to the `Azure/azure-sdk-assets` repo since the PAT does not grant write permission to that repo. Fortunately, we can request additional permissions through the `devcontainer.json`, which will give the Codespace write access to the `Azure/azure-sdk-assets` repo.

> Reference: https://docs.github.com/codespaces/managing-your-codespaces/managing-repository-access-for-your-codespaces

We need to apply this same `devcontainer` fix to _all_ our repos AND add a note within the test-proxy readme to reflect this small issue about PAT.

Timo's PR is [here](https://github.com/Azure/azure-sdk-for-js/pull/24963).