Increase size of file writing

[sosthene-nitrokey]

The increased size of files is a requirement for PIV, 1kB certificates are too small. This commit also removes the user_attribute field that was never read anyway. This avoids increasing the total size of the interchange

Both changes are breaking changes:

• Changing the messages size for the read and write requests can cause compilation error for code that depends on the length of the heapless arrays
• Removing user_attribute will require changing every call to the read_file and write_methods to remove the None.

[nickray]

Yeah, we can remove UserAttribute. If I remember correctly, it would have allowed filtering a tree of files without loading the files themselves, which might be an optimization for apps with a large amount of such files. So since we don't use it, can add back later if and only if someone actually needs this.

[sosthene-nitrokey]

This will we replaced by the streaming api. Current advances can be found in Nitrokey#18. Once #96 and trussed-dev/littlefs2#34 are merged I will upstream it.