kernel: Introduce empty and unused kernel::Context

[META] In the next commit, we will move the init::{Set,Unset}Globals
       logic into this struct.

Co-Authored-By: Ryan Ofsky <ryan@ofsky.org>

src/kernel/context.h

@@ -0,0 +1,20 @@
+// Copyright (c) 2022 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_KERNEL_CONTEXT_H
+#define BITCOIN_KERNEL_CONTEXT_H
+
+namespace kernel {
+//! Context struct holding the kernel library's logically global state, and
+//! passed to external libbitcoin_kernel functions which need access to this
+//! state. The kernel libary API is a work in progress, so state organization
+//! and member list will evolve over time.
+//!
+//! State stored directly in this struct should be simple. More complex state
+//! should be stored to std::unique_ptr members pointing to opaque types.
+struct Context {
+};
+} // namespace kernel
+
+#endif // BITCOIN_KERNEL_CONTEXT_H