feat: add before hooks for mutations

[zbeyens]

Summary

• add beforeCreate, beforeUpdate, and beforeDelete hooks so payloads can be transformed ahead of writes
• propagate the before hook handles through adapters and client trigger APIs
• add a changeset to release the new hook support

Testing

• pnpm typecheck

[changeset-bot]

🦋 Changeset detected

Latest commit: bfc2ad3

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
better-auth-convex	Minor

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