kernel: Add kernel library context object

The context introduced here holds the objects that will be required for
running validation tasks, such as the chosen chain parameters, callbacks
for validation events, and interrupt handling. These will be used by the
chainstate manager introduced in subsequent commits.

This commit also introduces conventions for defining option objects. A
common pattern throughout the C header will be:
```
options = object_option_create();
object = object_create(options);
```
This allows for more consistent usage of a "builder pattern" for
objects where options can be configured independently from
instantiation.

src/test/kernel/test_kernel.cpp

@@ -381,3 +381,17 @@ BOOST_AUTO_TEST_CASE(logging_tests)
     }
     Logger logger{std::make_unique<TestLog>()};
 }
+
+BOOST_AUTO_TEST_CASE(btck_context_tests)
+{
+    { // test default context
+        Context context{};
+        Context context2{};
+        CheckHandle(context, context2);
+    }
+
+    { // test with context options
+        ContextOptions options{};
+        Context context{options};
+    }
+}