[Feature] Add config vars for conversions

To support dynamic enablement of conversion-related tables in Facebook Ads and Reddit Ads, we need to add configuration variables to the models and also the `quickstart.yml` files.

When the conversions staging models are disabled, the corresponding end models should still run by filling in `NULL` values for the conversion fields. This ensures compatibility with downstream rollup reporting models and prevents model breakages due to missing fields.

### Facebook Ads:
- One variable that enables/disabled the following models:

source:
- basic_ad_actions
- basic_ad_action_values

transform
- int_facebook_ads__conversions
- facebook_ads__account_report
- facebook_ads__ad_report
- facebook_ads__ad_set_report
- facebook_ads__campaign_report
- facebook_ads__url_report

### Reddit Ads:
- It is probably more straightforward to also use one variable here, but one corresponding to each source could also be used.

source:
- account_conversions_report
- ad_group_conversions_report
- ad_conversions_report
- campaign_conversions_report

transform:
- reddit_ads__account_report
- reddit_ads__ad_group_report
- reddit_ads__ad_report
- reddit_ads__campaign_report
- reddit_ads__url_report