Add GET /public-api/opportunity/ext/{externalIdentifier} endpoint

Description: The ext/ pattern is currently implemented for write operations on opportunities (add/update, win, lose) but there is no equivalent GET endpoint. Customers who create opportunities via external system identifiers have no way to look up an opportunity by that identifier without fetching all opportunities and filtering client-side.

Requested by W.T. Solutions (Peter van der Waal).

Per dev confirmation this is straightforward to add — the pattern exists, it just wasn't implemented for GET due to lack of prior demand.

Acceptance criteria: GET /public-api/opportunity/ext/{externalIdentifier} returns the matching opportunity object, consistent with the existing GET by internal ID response.