Allow disabling agent auto upgrade

[antoinemzs]

Context

The agent is forcibly upgraded every time the version is bumped server-side.

Use case

Primarily a dev use-case: disabling the agent automatic upgrade client-side (on the agent, via configuration) would help test arbitrary compiled binaries on endpoints without having to worry mangling the server code.

Current Workaround

Mangle the server-side auto upgrade job lining.

Proposed Solution

A documented config key but stated as "advanced" or "reserved" , to ignore running upgrade jobs lined up by the server.