Add "overhead" duration to duration summary

I have some build scripts that do variable initialization, etc. Then I get this output:

```
--------------------------------------------------------------------------------
Initializing versioning
--------------------------------------------------------------------------------

No version info specified, falling back to GitVersion
No local .git directory found, treating as dynamic repository
Since dynamic repositories do not yet work correctly, we clear out the cloned temp directory (which is slow, but should be fixed in 5.0 beta)
Defined version: '3.2.0-alpha.77', commits since version source: '77'

--------------------------------------------------------------------------------
Initializing the state of the build
--------------------------------------------------------------------------------

IsAlphaBuild: True
IsBetaBuild: False
IsOfficialBuild: False
Channel: alpha
PublishType: Alpha
Running target 'Initialize'
Using output directory 'C:\CI_WS\Ws\133294\Output'
Validating input

========================================
Initialize
========================================

--------------------------------------------------------------------------------
Writing special values back to build server
--------------------------------------------------------------------------------

Setting version '3.2.0-alpha.77 ci' in Continua CI
Build version has been set to '3.2.0-alpha.77 ci'.
Setting variable 'channel' to 'alpha' in Continua CI
Variable '%channel%' has been set to 'alpha'.
Setting variable 'publishType' to 'Alpha' in Continua CI
Variable '%publishtype%' has been set to 'Alpha'.
Setting variable 'isAlphaBuild' to 'True' in Continua CI
Could not resolve variable 'isalphabuild' as it does not exist.
Setting variable 'isBetaBuild' to 'False' in Continua CI
Could not resolve variable 'isbetabuild' as it does not exist.
Setting variable 'isOfficialBuild' to 'False' in Continua CI
Variable '%isofficialbuild%' has been set to 'False'.

Task Duration
--------------------------------------------------
Initialize 00:00:00.0109852
--------------------------------------------------
Total: 00:00:00.0109852
```

As you can see, this looks like it's superfast (100 ms), but the initialization stage is what is taking the longest. Just an idea: what if there would be an "overhead" time category as well (code executed that was not part of a task)?

What I would like to know (if possible at all), is:

1. Time to compile scripts
2. Overhead / initialization code duration (initialization of variables that are not done inside a specific task)

No source file or test is identified in the issue. Start by locating the duration-summary and task-timing entry points, then trace how initialization and script compilation are measured. Done should include a clear overhead category in the summary, with coverage for the requested timing breakdown.

Written by the indexing model from the issue text.