Add the command execution context from the design: its api.h struct and vtable, plus the execCtx parameter on run. Implement it in the host on top of OperationContext, and add the SDK handle. Don't opt in to stepdown interrupts; the built-in search index commands don't either. Add a polling mode to the echo extension.

We should also investigate whether it is better to expose a single host member to handle this, and possibly unify with the agg stage API.