feat: Improve agent container start time by optionally caching downloaded agents onto an attached volume

[akanieski]

Agent container start times can be long anywhere from 10-60s. After analysis it seems that a fair bit of time is spent downloading a fresh copy of the agent binaries at runtime. This shouldn't have to happen every time.

Proposal
Update the sample start.ps1 and start.sh to first check if there is a /mnt/agent-cache and if there is check to see if the specific agent package has been downloaded already. If so just use that package instead of having to download the package at runtime.