Implement `getSchema` method for MySQL and PostgreSQL

[jverce]

WHY

To comply with the SQL prop interface, which uses the getSchema method in the app component to fetch metadata information about the current DB's schema (e.g. fields, data types, etc.).

This PR also renames all private methods in the app components so that we keep their interfaces clean and minimize unexpected usage.

[vercel[bot]]

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Name	Status	Preview	Comments	Updated (UTC)
pipedream-docs-redirect-do-not-edit	🔄 Building (Inspect)	Visit Preview	💬 Add feedback	Apr 23, 2024 8:41pm

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| Name | Status | Preview | Comments | Updated (UTC) | | :--- | :----- | :------ | :------- | :------ | | **docs-v2** | ⬜️ Ignored ([Inspect](https://vercel.com/pipedreamers/docs-v2/2zfuXWd4i8RHfM8REWTPN7dbQnWZ)) | [Visit Preview](https://docs-v2-git-fork-jverce-feat-mysql-for-sql-prop-pipedreamers.vercel.app) | | Apr 23, 2024 8:41pm |

[pipedream-component-development]

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[pipedream-component-development]

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