[heft] Add tryLoadProjectConfigurationFileAsync API and use in plugins

[dmichon-msft]

Summary

Adds a new API to HeftConfiguration, tryLoadProjectConfigurationFileAsync, so that plugins can load riggable config files without taking a direct dependency (and risking version duplication) on @rushstack/heft-config-file.

Starts addressing #5092. The motivation is to get us to a world where Heft plugins don't need any runtime dependencies other than on any external tools they bring with them, because all the core functionality of a Heft plugin is available on arguments passed to apply, similar to prior art in webpack plugins, that have the compiler.webpack field that provides access to all functionality that would otherwise be obtained via import ... from 'webpack'.

This API change removes any need for Heft plugins to take their own dependency on the @rushstack/heft-config-file package for normal scenarios.

Details

Fixes an issue with PathResolutionMethod.resolveRelativeToProjectRoot when extending from a file that lives in a different package. It was previously resolving relative to the package that contained the file being extended from, rather than the file that contains the original loaded configuration file.

Adds a new customValidationFunction option to the configuration file APIs that can be used to perform additional validation after loading a file.

Uses this API in heft-api-extractor-plugin, heft-sass-plugin, and heft-typescript-plugin.
Currently doesn't expose the version that throws if the file cannot be found.

All needed types are re-exported under the ConfigurationFile namespace on @rushstack/heft.

How it was tested

Existing build tests for these plugins.

Impacted documentation

API Documentation for @rushstack/heft.