Add in-band sync capability

[amh4r]

Description

Add the "in-band sync" capability. This capability is inactive unless the user explicitly allows it via a new INNGEST_ALLOW_IN_BAND_SYNC env var.

Maintainers: To read more about in-band syncing, refer to our internal spec here.

Changes

• Update register function to conditionally perform an in-band or out-of-band sync.
  • inBandSync is a new function that contains our in-band sync logic.
  • outOfBandSync contains the same logic that register used to have, but function config building is extracted to createFunctionConfigs.
• Add INNGEST_ALLOW_IN_BAND_SYNC env var to control whether in-band syncing is allowed. It defaults to false for now, but will default to true when in-band syncing is stable.
• Update OSS to 4c207d8fb0ce.