validationcaches: Add and use ValidationCacheSizes

Also:

- Make DEFAULT_MAX_SIG_CACHE_SIZE into constexpr
  DEFAULT_MAX_SIG_CACHE_BYTES to utilize the compile-time integer
  arithmetic overflow checking available to constexpr.
- Fix comment (MiB instead of MB) for DEFAULT_MAX_SIG_CACHE_BYTES.
- Pass in max_size_bytes parameter to InitS*Cache(), modify log line to
  no longer allude to maxsigcachesize being split evenly between the two
  validation caches.
- Fix possible integer truncation and add a comment.

[META] I've kept the integer types as int64_t in order to not introduce
       unintended behaviour changes, in the next commit we will make
       them size_t.

src/kernel/validation_cache_sizes.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_VALIDATION_CACHE_SIZES_H
+#define BITCOIN_KERNEL_VALIDATION_CACHE_SIZES_H
+
+#include <script/sigcache.h>
+
+#include <cstdint>
+#include <limits>
+
+namespace kernel {
+struct ValidationCacheSizes {
+    int64_t signature_cache_bytes{DEFAULT_MAX_SIG_CACHE_BYTES / 2};
+    int64_t script_execution_cache_bytes{DEFAULT_MAX_SIG_CACHE_BYTES / 2};
+};
+}
+
+#endif // BITCOIN_KERNEL_VALIDATION_CACHE_SIZES_H