feat: Add createOriginMiddleware function

[FrederikBolding]

Explanation

Add a createOriginMiddleware function which adds the origin to the MiddlewareContext, to be leveraged by later middlewares.

References

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

---

Note

Low Risk
Low risk: adds a small new middleware helper and export without changing existing request handling behavior.

Overview
Adds a new createOriginMiddleware(origin) utility for JsonRpcEngineV2 that sets context.origin and then delegates to next().

Exports the helper from @metamask/json-rpc-engine/v2, adds a unit test to validate context propagation, and updates the package changelog to announce the new API.

^{Reviewed by Cursor Bugbot for commit bacdcef. Bugbot is set up for automated code reviews on this repo. Configure here.}

[rekmarks]

Literally perfect