Instrumenting build retrieval and unpacking times

[vitorguidi]

Motivation

We currently lack metrics for build retrieval and unpacking times. This PR adds that, with granularity by fuzz target and job type.

There are three different implementations for build downloading/unpacking:

• In the Build class, from which RegularBuild, SplitTargetBuild, FuchsiaBuild and SymbolizedBuild inherit the downloading/unpacking behavior
• In the CustomBuild class, which implements its own logic

There are two possible cases for downloading/unpacking: clusterfuzz either downloads the whole build and unpacks it locally, or unpacks it remotely. This is the case for all build types except CustomBuild, which does not perform remote unpacking.

Part of #4271