API: Add POST and GET-by-ext-ID endpoints for opportunities, and allow setting externalIdentifier on existing record by internal ID

Description:

The current Public API has a gap when Salesbuildr is the originating system for opportunities. When an opportunity is created in Salesbuildr without an externalIdentifier, there is no way to assign one after the fact, and no way to create or retrieve opportunities using an external ID directly.

Missing endpoints:

POST /public-api/opportunity/ext/{externalIdentifier} — create an opportunity with an external ID set from the start

GET /public-api/opportunity/ext/{externalIdentifier} — retrieve an opportunity by external ID

A way to set or update the externalIdentifier on an existing opportunity using its internal Salesbuildr ID (e.g. PATCH /public-api/opportunity/{id})

Use case:

We create opportunities in Salesbuildr and sync them to an external system via webhook. The external system assigns its own ID and needs to write it back to Salesbuildr so future updates can use the ext/ path. Without these endpoints, the bidirectional link cannot be established when Salesbuildr is the record origin.

Impact:

Blocks bidirectional integration patterns where Salesbuildr is the originating system. The existing ext/ endpoints are effectively unusable for opportunities not created through the API.