e2e Go-native test runner

Rewrite the `e2e-tests.sh` script (https://github.com/knative/hack/blob/main/README.md#using-the-e2e-testssh-helper-script) into a Golang tool `knative.dev/toolbox/presubmit`.

Invent new Go-native extension interface, for example:

```sh
# will look for Go files with `e2e` build constraints, that register configuration
go run knative.dev/toolbox/e2e@latest --run-tests
```

An example configuration might look like:

```go
//go:build e2e
package test

import (
"fmt"
"os"

"github.com/bitfield/script"
"knative.dev/toolbox/e2e/pkg/config"
"knative.dev/toolbox/e2e/pkg/commands/knative"
"knative.dev/toolbox/e2e/pkg/commands/wait"
)

var waitForKnative = true

init {
config.Configure(config.Hooks{
KnativeSetup: function(ctx config.Context) error {
if err := knative.StartServingLatest(); err != nil {
return err
}
if waitForKnative {
return wait.UntilPodsRunning("knative-serving")
}
return nil
},
DefineFlags: function(fs *flag.FlagSet) {
fs.BoolVar(&waitForKnative, "wait-for-knative", true, "wait for Knative install?")
},
Tests: []config.E2eSuite{{
Name: "Istio",
Dir: "./...",
Flags: ["--istio"],
}},
})
}
```

Start with the e2e-tests.sh helper described in the Knative hack README, then review the proposed knative.dev/toolbox/presubmit tool and the example Go-native extension interface. Done means the existing helper workflow is represented by a Go tool with e2e build-constraint configuration, hooks, flags, and test suites as outlined in the issue.

Written by the indexing model from the issue text.