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/Makefile.am

@@ -177,6 +177,7 @@ BITCOIN_CORE_H = \
   kernel/mempool_limits.h \
   kernel/mempool_options.h \
   kernel/mempool_persist.h \
+  kernel/validation_cache_sizes.h \
   key.h \
   key_io.h \
   logging.h \
@@ -207,6 +208,7 @@ BITCOIN_CORE_H = \
   node/psbt.h \
   node/transaction.h \
   node/utxo_snapshot.h \
+  node/validation_cache_args.h \
   noui.h \
   outputtype.h \
   policy/feerate.h \
@@ -390,6 +392,7 @@ libbitcoin_node_a_SOURCES = \
   node/minisketchwrapper.cpp \
   node/psbt.cpp \
   node/transaction.cpp \
+  node/validation_cache_args.cpp \
   noui.cpp \
   policy/fees.cpp \
   policy/fees_args.cpp \