feat: add support for loading multiple config files and staleness check

[Elia-Renzoni]

@jranson @crandles
This PR improves Trickster’s configuration system by extending support from a single configuration file to multiple YAML configuration files loaded from a directory.

The implementation assumes that users who want to use multiple configuration files will provide only the directory path as a flag
(e.g., /etc/trickster) and the code will automatically look for all YAML files in that directory and load them in order, merging the configurations.

To support this feature, some methods were refactored, any feedback on these changes would be appreciated.

Related Issue: #493

[coveralls]

Pull Request Test Coverage Report for Build 20577480854

Details

• 65 of 87 (74.71%) changed or added relevant lines in 2 files are covered.
• 8 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.04%) to 72.087%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/config/config.go	64	86	74.42%

Files with Coverage Reduction	New Missed Lines	%
pkg/proxy/engines/cache_read.go	2	81.82%
pkg/proxy/engines/progressive_collapse_forwarder.go	2	96.52%
pkg/config/config.go	4	73.72%

Totals
Change from base Build 20349547424:	-0.04%
Covered Lines:	14958
Relevant Lines:	20750

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💛 - Coveralls

[jranson]

@Elia-Renzoni thanks so much for this, we haven't forgotten about it. We were already feature-locked and in the release candidate phase for v2.0.0 when this PR was opened. We are targeting this for v2.1.0. I'll review and test it out this week. Thanks again!