Talk about the underlying buffer data structure of the BloomFilter

[zncleon]

Currently, the underlying buffer data structure of the BloomFilter is nonstd::span<char>[1], but in some scenes, it maybe not suitable.
For example, the PinnableSlice .data() return the const char*, which can't fit the BlockSplitBloomFilter for the data type is span<char> and we need span<const char>. I have 2 solutions to temporarily resolve it:

1. Use template span<T> and specialize span<char> and span<const char>
2. Just use const_cast<char *> to converse the PinnableSlice .data() return

But I think the two way is not graceful and reasonable.
If we should introduce a widely suitable data structure[2], which ont only contains the pointer to the data but also contains the is_mutable, is_owned and other member to fit in more scenes.

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[1] https://github.com/apache/kvrocks/blob/unstable/src/types/bloom_filter.h
[2] https://github.com/apache/arrow/blob/main/cpp/src/arrow/buffer.h

[mapleFU]

How about just hide the mutable inside the BloomFilter(without adding a Buffer type), and use const_cast internal? 🤔

Also cc @PragmaTwice