Allow 3 bytes of segment padding in object fetcher

[teor2345]

The old code for the object fetcher assumed the maximum segment padding was 2 bytes, but it is actually 3 bytes. (Assuming objects are limited to < 1 GB.)

This PR fixes that bug, and returns an error for objects larger than 1 GB. Currently the maximum object size is 5 MB, because object size is limited by domain block size.

If we need to support larger objects, we can update the code to allow more padding easily.

Close #3324.

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[nazar-pc]

Non-blocking

[nazar-pc]

Why .unwrap_or(usize::MAX) and not .unwrap_or(MAX_SUPPORTED_OBJECT_LENGTH)?

Also I'd prefer making max_object_len non-optional, this is getting too confusing (it accepts the input, but actually limits the size internally).

[nazar-pc]

What is the reason for this, why can't caller specify whatever limit they want?

[teor2345]

The current implementation assumes the largest compact encoded object length is 4 bytes, or a 1 GB - 1 length:

subspace/shared/subspace-data-retrieval/src/object_fetcher.rs

Lines 29 to 34 in b856cce

	/// The maximum amount of segment padding.
	///
	/// This is the difference between the compact encoding of lengths 1 to 63, and the compact
	/// encoding of lengths 2^14 to 2^30 - 1.
	/// <https://docs.substrate.io/reference/scale-codec/#fn-1>
	pub const MAX_SEGMENT_PADDING: usize = 3;

We can change that assumption any time we like, but it seems pointless right now, because the largest current block size in any domain is 5 MB.