Source Facebook Marketing: add 'monthly' to time_increment for Custom Insights

## Problem

The Facebook Marketing API supports several `time_increment` values for Insights queries: integers 1-90 (days), `monthly`, and `all_days`.

The connector's `InsightConfig.time_increment` in `spec.py` currently accepts `"1"`, `"7"`, `"28"`, and `"all_days"` — but `"monthly"` is missing, even though the Facebook API handles it natively.

Monthly aggregation returns data grouped by calendar month with deduplicated unique metrics like reach — which cannot be accurately derived by summing daily values.

## Proposed Fix

Add `"monthly"` as an accepted `Literal` value for `time_increment` in `InsightConfig` in [spec.py](https://github.com/airbytehq/airbyte/blob/master/airbyte-integrations/connectors/source-facebook-marketing/source_facebook_marketing/spec.py). No changes needed in the API request logic since Facebook already supports this value.

Additionally, the Airbyte Cloud API schema validates `time_increment` as `integer`, rejecting all string values — including the already-supported `"all_days"`. This should be updated to match the connector's actual spec.

## References

- [Facebook API docs — time_increment](https://developers.facebook.com/docs/marketing-api/insights/parameters/v22.0)
- #14391 — Added `all_days` support (closed/completed)
- #57006 — Cloud API rejects string values for `time_increment` (closed as stale)

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**Internal Tracking:** https://github.com/airbytehq/oncall/issues/11537