[RFC]: "link" command for accelerating remote cache setup

Overview

Currently, there is no command that one can use to set up remote cache. The built-in remote cache provider is "Azure Blob Storage". However, it is through backfill's documentation (readme) that users get a HINT on how to set up remote cache. The documentation also does not cover how a developer of a repo would be able to access the remote cache as read-only blobs.

Goals & non-goals

Goals

1. give users a single command to

• sign in to azure cli
• create or choose a blob container
• save the R/O connection string in lage.config.js
• give users a command to retrieve & display the correct environment variables for ADO pipelines

2. document this command in the docs site

• in the tutorial section (?)
• in the cookbooks section as "Remote Cache with Azure Blob Storage"

Upstream dependencies

1. backfill
2. azure-cli potentially

Test Plan

• using a mocked azure cli interface, we should create snapshots of expected outputs of the hint messages
• unit tests should be employed to make sure the connection string generation is correct

Performance, Resilience, Monitoring

• we need to validate the version of the upstream dep of azure-cli before executing it

Security & Privacy

• we should NEVER store the readonly blob storage SAS token - the expiry should be set to a relatively short period so as to make sure we can ensure it will not expire during a build, but short enough that the compromised token would not last long
• read-write tokens are to be generated MANUALLY (access keys to blob container) and stored as part of the standard pipeline secrets mechanism (keyvault, etc)