feat(providers): add CoinGecko provider for real-time and historical crypto data

[Shreyansh1729]

Resolves #7177.

Description

This PR integrates the free CoinGecko Public API as a new data provider (openbb_coingecko) to address the requested enhancement for additional real-time cryptocurrency data sources.

Current integration status:

• Minimal viable provider scaffold added.
• CryptoHistorical data fetcher implemented using the /simple/price or /market_chart endpoints.

Can be expanded easily with more endpoints, but provides the coingecko namespace and foundational dependency logic.

[deeleeramone]

Thanks for the PR. By itself, historical OHLC doesn't really add anything of value. For this type of provider to be included here, it would need to implement the full API surface, or at least the Demo API, and also be pluggable for people with real credentials and subscriptions.

https://docs.coingecko.com/v3.0.1/reference/authentication
https://docs.coingecko.com/reference/authentication

If you want to plug away at that, we should mark this PR as a draft and I can assign the issue to you. Let me know what you want to do!