Fix Win32 Narrowing Error

Fix
```
2025-01-16T05:51:21.6362891Z D:\a\openPMD-api\openPMD-api\src\dep-adios2\ADIOS2-2.10.2\source\adios2\toolkit\format\bp5\BP5Serializer.cpp(1603,68): error C2397: conversion from 'uint64_t' to 'size_t' requires a narrowing conversion [D:\a\openPMD-api\openPMD-api\src\build-adios2\source\adios2\adios2_core.vcxproj]
2025-01-16T05:51:21.6371102Z D:\a\openPMD-api\openPMD-api\src\dep-adios2\ADIOS2-2.10.2\source\adios2\toolkit\format\bp5\BP5Serializer.cpp(1611,69): error C2397: conversion from 'uint64_t' to 'size_t' requires a narrowing conversion [D:\a\openPMD-api\openPMD-api\src\build-adios2\source\adios2\adios2_core.vcxproj]
```
with MSVC on Win32 ILP32.

`core::iovec` is a `void*` and `size_t`, so this part needs to be `size_t` as well.

source/adios2/toolkit/format/bp5/BP5Serializer.cpp

@@ -1615,17 +1615,17 @@ std::vector<core::iovec> BP5Serializer::BreakoutContiguousMetadata(
             }
         }
         helper::CopyFromBuffer(Aggregate.data(), Position, &MBCount);
-        for (uint64_t i = 0; i < MBCount; ++i)
+        for (size_t i = 0; i < MBCount; ++i)
         {
-            uint64_t MEBSize;
+            size_t MEBSize;
             helper::CopyFromBuffer(Aggregate.data(), Position, &MEBSize);
             MetadataBlocks.push_back({Aggregate.data() + Position, MEBSize});
             Position += MEBSize;
         }
         helper::CopyFromBuffer(Aggregate.data(), Position, &ABCount);
-        for (uint64_t i = 0; i < ABCount; ++i)
+        for (size_t i = 0; i < ABCount; ++i)
         {
-            uint64_t AEBSize;
+            size_t AEBSize;
             helper::CopyFromBuffer(Aggregate.data(), Position, &AEBSize);
             AttributeBlocks.push_back({Aggregate.data() + Position, AEBSize});
             Position += AEBSize;