CLM user code location callback alternative to LocationOverride option

[nr-swilloughby]

Fixes Issue #640.
Deprecates the CLM option to set LocationOverride, which forces the caller to do the work to determine the code location up-front even if CLM is disabled. Replaces this with a new function which registers a callback function, which the CLM code will call to get the function location from the user's code only if actually needed. The user's callback may use the existing caching code location functions to further optimize this.

[codecov-commenter]

Codecov Report

Attention: 15 lines in your changes are missing coverage. Please review.

Comparison is base (e98ff73) 80.33% compared to head (932f589) 80.23%.
Report is 4 commits behind head on develop.

Files	Patch %	Lines
v3/newrelic/code_level_metrics.go	37.50%	8 Missing and 2 partials ⚠️
v3/newrelic/secure_agent.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #838      +/-   ##
===========================================
- Coverage    80.33%   80.23%   -0.10%     
===========================================
  Files          139      139              
  Lines        12603    12621      +18     
===========================================
+ Hits         10124    10127       +3     
- Misses        2189     2202      +13     
- Partials       290      292       +2     

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