[api-extractor] Support customizing defaults when loading config

[dmichon-msft]

Summary

Adds the ability for tools consuming API-Extractor to customize the default values when loading config files.

Details

Provided custom defaults override global defaults, then the config file values are written on top of them.   Uses this ability in heft-api-extractor-plugin to change the default API report temp folder in the Heft task temp folder.

How it was tested

Added unit test for config file custom defaults. Validated new location for temp api.md file in rushstack projects.

Impacted documentation

Currently none, unless something mentions the path to which heft-api-extractor-plugin writes its temp files.