Add polygon.io provider

[monstrobishi]

What would you like to be added:

Add provider for https://polygon.io/ , to follow existing provider pattern.

Why is this needed:

Polygon.io is a good source for price feeds.

[defichain-bot]

@monstrobishi: Thanks for opening an issue, it is currently awaiting triage.

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[defichain-bot]

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Details

I am a bot created to help the DeFiCh developers manage community feedback and contributions. You can check out my manifest file to understand my behavior and what I can do. If you want to use this for your project, you can check out the DeFiCh/oss-governance-bot repository.

[canonbrother]

fix on BirthdayResearch/oracle-adapters#87