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 because this PR mainly adds a new, currently no-op package and wires it into repo metadata/build config without changing existing runtime behavior.

Overview
Introduces a new package, @metamask/snap-account-service, scaffolding the SnapAccountService messenger-based service (currently with an empty init() and no exposed actions/events) plus a basic Jest test and package docs.

Wires the new package into monorepo plumbing: adds it to root README package list/dependency graph, CODEOWNERS/teams.json ownership, root tsconfig references, and yarn.lock workspace entry, along with standard package configs (TypeScript, Jest, Typedoc, changelog, license).

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[mcmire]

LGTM!