Add API support to look up Sentry issues by linked Jira issue

### Problem Statement

Reported via [Zendesk ticket](): Right now, the API only lets you [fetch external issues ]() for a selected Sentry issue, but not the other way around. Currently, there isn't a way to query Sentry issues by a Jira issue ID. It’d be helpful to have an endpoint that returns all Sentry issues linked to a specific Jira ticket.

### Solution Brainstorm

*No response*

### Product Area

Settings - Integrations

We want to define a set of [lightweight metrics]() that SDKs can automatically collect for e.g. framework, language or platform, focusing only on signals that **do NOT overlap with Tracing**. Ideally these metrics should give a quick sense of app or runtime health, … while deeper investigation will still rely on traces or profiles.

Examples**:**

* Node.js runtime metrics (e.g., event loop delay, heap usage)
* Prisma-related metrics in Next.js
* Mobile CPU / memory usage to help surface potential ANRs or app hangs

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**Candidate Metrics (add more below)**

* …

*(please also add ideas, questions or examples in comments)*

Start by reviewing the existing external-issue lookup API linked in the issue and the Sentry integration settings area; no implementation files or tests are named. Done means an API endpoint can accept a Jira issue ID and return all linked Sentry issues, with coverage for the expected lookup behavior.

Written by the indexing model from the issue text.