all: statically enforce json/v2 interface satisfaction (#15154)

The json/v2 prototype is still in flux and the API can/will change.

Statically enforce that types implementing the v2 methods
satisfy the correct interface so that changes to the signature
can be statically detected by the compiler.

Updates tailscale/corp#791

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

types/opt/value_test.go

@@ -13,6 +13,11 @@ import (
 	"tailscale.com/util/must"
 )
 
+var (
+	_ jsonv2.MarshalerTo     = (*Value[bool])(nil)
+	_ jsonv2.UnmarshalerFrom = (*Value[bool])(nil)
+)
+
 type testStruct struct {
 	Int int    `json:",omitempty,omitzero"`
 	Str string `json:",omitempty"`