feat(sharedb): add submit middleware hooks

[edocevol]

Summary

• add a submit middleware API for sharedb servers
• allow middleware to reject, wrap, and rewrite structured submit requests
• document the new hook and add tests for rejection, ordering, rewriting, and nil-handler safety

Test Plan

• go test ./... (repo root)
• cd sharedb && go test ./...
• cd sharedb && go vet ./...
• cd sharedb && go test -race ./...