feat(shared-ini-file-loader): refactor loadSharedConfigFiles into modular components

[trivikr]

Issue

Fixes: #3019
Refs: #3279

Description

• Refactors loadSharedConfigFiles to break it into individual components.
• Caches contents of readFile for a path, and ensures that it's called only once in parallel calls.

Testing

• Unit testing
• yarn test:integration
• yarn test:integration:legacy

Single client

Refs: #3019 (comment)

• Before: 4 calls to readFile
• After: 2 calls to readFile

Two clients

Refs: #3019 (comment)

• Before: 8 calls to readFile
• After: 2 calls to readFile

Client with an operation

Refs: #3019 (comment)

• Before: 16 calls to readFile
• After: 2 calls to readFile

Additional Context

This is the merge of prior attempts in:

• feat(shared-ini-file-loader): remove concurrent/duplicate calls to readFile #3281
• feat(shared-ini-file-loader): use promise callback queue on loadSharedConfigFiles #3282

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[AllanZhengYP]

Looks good! Thank you for taking care of it!

[trivikr]

npm pack size comparison:

Before: 7.4 kB

$ yarn clean && rm -rf *.tsbuildinfo && yarn build

$ npm pack
...
npm notice package size:  7.4 kB                                    
npm notice unpacked size: 23.4 kB
...

After: 8.5 kB

$ yarn clean && rm -rf *.tsbuildinfo && yarn build

$ npm pack
...
npm notice package size:  8.5 kB                                    
npm notice unpacked size: 27.8 kB
...

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