custom check functions beyond http get

## summary
support check types beyond http GET — tcp, grpc, and arbitrary user-defined functions.

## details
- introduce a `CheckFunc` type (e.g. `func(ctx context.Context) error`)
- add built-in helpers: `TCPCheck(addr)`, `GRPCCheck(addr)`
- allow `WithTargetFunc(name string, fn CheckFunc)` alongside the existing `WithTarget`
- keep the current http get behaviour as the default