feat: Add legacy createOriginMiddleware

[FrederikBolding]

Explanation

Recently we added createOriginMiddleware to v2 for usage in the clients. Though using it via asLegacyMiddleware introduces some complications, so for the time being we introduce the same middleware using the legacy API.

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

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Note

Low Risk
Low risk additive change: introduces a new legacy middleware and export without altering existing middleware behavior, with a small chance of conflicts for consumers that already attach an origin field to requests.

Overview
Adds a legacy createOriginMiddleware for @metamask/json-rpc-engine that mutates the JSON-RPC request object to include an origin string (marked deprecated in favor of the v2 API).

Exports the new utility from the package entrypoint, updates export snapshot tests, adds a unit test verifying origin is set, and records the addition in the package changelog.

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