[tokens-controller] Expose `getState` action and `stateChange` event

[MajorLift]

Explanation

References

• Closes #4271

Changelog

@metamask/package-a

• <CATEGORY>: Your change here
• <CATEGORY>: Your change here

@metamask/package-b

• <CATEGORY>: Your change here
• <CATEGORY>: Your change here

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 highlighted breaking changes using the "BREAKING" category above as appropriate