feat(snap-account-service): add SnapAccountService

[ccharly]

Explanation

A new service where we will move the common logic of the current monolith SnapKeyring.

The idea is to be able to split this keyring into multiple ones and move the KeyringEvents logic in this service.

High-level architecture is the following:

For now, it's just an empty service, but we will move forward with more follow-up change as we are rewriting the existing SnapKeyring to use the new KeyringV2.

References

N/A

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: this PR primarily adds a new package scaffold with an effectively no-op SnapAccountService and wires it into monorepo configs, with no behavioral changes to existing packages.

Overview
Adds a new @metamask/snap-account-service package, including an initial SnapAccountService class (messenger-namespaced, no exposed actions/events yet) and a basic init() placeholder plus a minimal Jest test.

Wires the new package into repo metadata and build tooling: updates README.md package list/dependency graph, adds CODEOWNERS/teams.json ownership entries, and includes the package in root tsconfig references and yarn.lock workspace resolution.

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