tailcfg: add NodeCapMap

Like PeerCapMap, add a field to `tailcfg.Node` which provides
a map of Capability to raw JSON messages which are deferred to be
parsed later by the application code which cares about the specific
capabilities. This effectively allows us to prototype new behavior
without having to commit to a schema in tailcfg, and it also opens up
the possibilities to develop custom behavior in tsnet applications w/o
having to plumb through application specific data in the MapResponse.

Updates #4217

Signed-off-by: Maisem Ali <maisem@tailscale.com>

cmd/derper/depaware.txt

@@ -169,6 +169,7 @@ tailscale.com/cmd/derper dependencies: (generated by github.com/tailscale/depawa
         golang.org/x/crypto/nacl/box                                 from tailscale.com/types/key
         golang.org/x/crypto/nacl/secretbox                           from golang.org/x/crypto/nacl/box
         golang.org/x/crypto/salsa20/salsa                            from golang.org/x/crypto/nacl/box+
+        golang.org/x/exp/maps                                        from tailscale.com/tailcfg
    L    golang.org/x/net/bpf                                         from github.com/mdlayher/netlink+
         golang.org/x/net/dns/dnsmessage                              from net+
         golang.org/x/net/http/httpguts                               from net/http

cmd/tailscale/depaware.txt

@@ -175,7 +175,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         golang.org/x/crypto/pbkdf2                                   from software.sslmate.com/src/go-pkcs12
         golang.org/x/crypto/salsa20/salsa                            from golang.org/x/crypto/nacl/box+
    W    golang.org/x/exp/constraints                                 from github.com/dblohm7/wingoes/pe
-        golang.org/x/exp/maps                                        from tailscale.com/cmd/tailscale/cli
+        golang.org/x/exp/maps                                        from tailscale.com/cmd/tailscale/cli+
         golang.org/x/net/bpf                                         from github.com/mdlayher/netlink+
         golang.org/x/net/dns/dnsmessage                              from net+
         golang.org/x/net/http/httpguts                               from net/http+

cmd/viewer/viewer.go

@@ -237,9 +237,10 @@ func genView(buf *bytes.Buffer, it *codegen.ImportTracker, typ *types.Named, thi
 				slice := u
 				sElem := slice.Elem()
 				switch x := sElem.(type) {
-				case *types.Basic:
+				case *types.Basic, *types.Named:
+					sElem := it.QualifiedName(sElem)
 					args.MapValueView = fmt.Sprintf("views.Slice[%v]", sElem)
-					args.MapValueType = "[]" + sElem.String()
+					args.MapValueType = "[]" + sElem
 					args.MapFn = "views.SliceOf(t)"
 					template = "mapFnField"
 				case *types.Pointer: