FR: Add CopyBytesFrom for array types

```struct FileHeader:
0 [+8] UInt:8[8] identification_pattern
```

Ideally I'd be able to:

```
pw::Result result =
MakeEmbossWriter(file_header_data);
if (!result.ok()) {
return;
}
emboss::FileHeaderWriter writer = result.value();
std::array identification_pattern_data = {0x62, 0x74, 0x73,
0x6E, 0x6F, 0x6F, 0x70, 0x00};
writer.identification_pattern().CopyBytesFrom(identification_pattern_data);
```