ssh: stream management api (#5670)

## Summary

This implements the StreamManagement API defined at 

https://github.com/pomerium/envoy-custom/blob/main/api/extensions/filters/network/ssh/ssh.proto#L46-L60.
Policy evaluation and authorization logic is stubbed out here, and
implemented in https://github.com/pomerium/pomerium/pull/5665.

## Related issues

<!-- For example...
- #159
-->

## User Explanation

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change doesn't create any user-facing changes, you can leave
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## Checklist

- [ ] reference any related issues
- [ ] updated unit tests
- [ ] add appropriate label (`enhancement`, `bug`, `breaking`,
`dependencies`, `ci`)
- [ ] ready for review

pkg/ssh/manager_test.go

@@ -0,0 +1,40 @@
+package ssh_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.uber.org/mock/gomock"
+
+	extensions_ssh "github.com/pomerium/envoy-custom/api/extensions/filters/network/ssh"
+	"github.com/pomerium/pomerium/config"
+	"github.com/pomerium/pomerium/pkg/ssh"
+	mock_ssh "github.com/pomerium/pomerium/pkg/ssh/mock"
+)
+
+func mustParseWeightedURLs(t *testing.T, urls ...string) []config.WeightedURL {
+	wu, err := config.ParseWeightedUrls(urls...)
+	require.NoError(t, err)
+	return wu
+}
+
+func TestStreamManager(t *testing.T) {
+	ctrl := gomock.NewController(t)
+	auth := mock_ssh.NewMockAuthInterface(ctrl)
+	m := ssh.NewStreamManager(auth)
+
+	cfg := &config.Config{Options: config.NewDefaultOptions()}
+	cfg.Options.Policies = []config.Policy{
+		{From: "ssh://host1", To: mustParseWeightedURLs(t, "ssh://dest1:22")},
+		{From: "ssh://host2", To: mustParseWeightedURLs(t, "ssh://dest2:22")},
+	}
+
+	t.Run("LookupStream", func(t *testing.T) {
+		assert.Nil(t, m.LookupStream(1234))
+		sh := m.NewStreamHandler(cfg, &extensions_ssh.DownstreamConnectEvent{StreamId: 1234})
+		assert.Equal(t, sh, m.LookupStream(1234))
+		sh.Close()
+		assert.Nil(t, m.LookupStream(1234))
+	})
+}