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
Mostly adds a new, unused package scaffold and monorepo wiring; no business logic changes beyond a stub init() and a basic test.

Overview
Adds a new package, @metamask/snap-account-service, as the initial scaffold for a messenger-based SnapAccountService (currently no exposed actions/events and init() is a stub) plus a minimal Jest test that init() resolves.

Wires the new workspace into the monorepo (root tsconfig references, yarn.lock workspace entry, CODEOWNERS/teams.json ownership, and README package list/dependency graph), and includes standard package metadata/docs (README, changelog, license, typedoc/jest/tsconfig configs).

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