Feature Request: Dynamic dropdown for account/resource IDs in connector configuration

## Summary

Users currently have to manually find and enter account/resource IDs (e.g., Facebook Ad Account IDs, Google Ads Customer IDs, YouTube Channel IDs) when configuring connectors. This creates significant UX friction during onboarding.

**Requested by:** teo@airbyte.io

## Problem Statement

When configuring connectors like Facebook Marketing, users must:
1. Leave Airbyte
2. Navigate to external platforms (e.g., Meta Ads Manager)
3. Locate their account/page/channel IDs
4. Manually copy-paste them into Airbyte configuration forms

This is error-prone and creates a poor onboarding experience, especially for non-technical users.

## Proposed Solution

After OAuth authentication completes, Airbyte should:
1. Make an API call using the obtained credentials to fetch available accounts/resources
2. Populate a dropdown with the results
3. Allow users to select from the fetched options instead of manual entry

## Technical Analysis

Show/Hide Detailed Technical Analysis

### API Capability (Facebook Marketing Example)

The Facebook Graph API supports fetching ad accounts via `GET /me/adaccounts`. This endpoint is already referenced in the Facebook Marketing connector test fixtures (`conftest.py`).

### Current Limitation

Airbyte connector specifications are static JSON Schema. There is no platform-level mechanism to:
1. Trigger an API call after OAuth completes
2. Dynamically populate a dropdown with the results
3. Let users select from fetched options

### Existing Pattern (Instagram Connector)

The Instagram connector demonstrates runtime account discovery:
- Has an "Api" stream that calls `GET /me/accounts`
- Uses it as a parent stream via `SubstreamPartitionRouter`
- Dynamically discovers accounts at sync time

However, this happens during sync execution, not during configuration, so it does not solve the UX problem.

### What Would Be Needed

1. **Platform Enhancement**: Add support for "dynamic enum" or "dependent fields" in connector specs that can:
- Trigger after OAuth completes
- Make an API call using the obtained credentials
- Populate field options from the response

2. **Connector Updates**: Modify affected connectors to use this new capability

## Affected Connectors

This is a common pain point across many connectors that require account/workspace/project IDs:
- Facebook Marketing (Ad Account IDs)
- Google Ads (Customer IDs)
- YouTube Analytics (Channel IDs)
- Instagram (Business Account IDs)
- LinkedIn Ads (Account IDs)
- And many others...

## Impact

- **User Experience**: Significantly reduces friction during connector setup
- **Error Reduction**: Eliminates manual copy-paste errors
- **Onboarding**: Improves time-to-first-sync for new users

## Additional Context

This feature request originated from user feedback about the difficulty of connecting Facebook/Meta data sources. The user specifically mentioned: "We are encountering a massive amount of friction here."

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**Internal Tracking:** https://github.com/airbytehq/oncall/issues/10885