feat: add function runtime to dig.CallbackInfo

[tchung1118]

This change adds runtime of the associated constructor
or decorator to dig.CallbackInfo.

For example, users can access the runtime of a particular
constructor by:

c := dig.New()
c.Provide(NewFoo, dig.WithProviderCallback(func(ci dig.CallbackInfo) {
    if ci.Error == nil {
        fmt.Printf("constructor %q finished running in %v", ci.Name, ci.Runtime)
    }
}))

This change is a prerequisite for adding uber-go/fx#1213
to report runtime of constructors in Run events.

[codecov]

Codecov Report

Attention: Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.35%. Comparing base (897df36) to head (1ed1d62).
Report is 1 commits behind head on master.

❗ Current head 1ed1d62 differs from pull request most recent head 26d26ec

Please upload reports for the commit 26d26ec to get more accurate results.

Files	Patch %	Lines
container.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #412      +/-   ##
==========================================
- Coverage   98.40%   98.35%   -0.05%     
==========================================
  Files          22       23       +1     
  Lines        1507     1524      +17     
==========================================
+ Hits         1483     1499      +16     
- Misses         15       16       +1     
  Partials        9        9              

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[JacobOaks]

LGTM modulo some small nits.

Can you also put an example of how this is accessed in the PR summary?