feat: Add DecOptions.FixedArrayLength to enforce Go array length

[AdamMagued]

Description

When decoding CBOR byte strings or arrays into Go fixed-size arrays ([N]byte or [N]T), the default decoder behavior follows Go standard library encoding/json semantics: shorter inputs are padded with zero values and longer inputs are truncated.

This PR introduces a FixedArrayLength decoding option (FixedArrayLengthMode / alias ArrayLengthMode) with two settings:

• FixedArrayLengthNone (default): maintains standard library compatibility by zero-padding shorter inputs and discarding extra elements.
• FixedArrayLengthEnforced: strictly validates that the number of bytes or elements in the CBOR data item matches the dimension of the target Go array, returning *UnmarshalTypeError when there is a mismatch.

Fixes #735

PR Was Proposed and Welcomed in Currently Open Issue

• This PR was proposed and welcomed by maintainer(s) in issue bug: decoding fixed bytes into fixed array #735
• Closes or Updates Issue bug: decoding fixed bytes into fixed array #735

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[fxamacker]

When decoding CBOR byte strings or arrays into Go fixed-size arrays ([N]byte or [N]T), the default decoder behavior follows Go standard library encoding/json semantics: shorter inputs are padded with zero values and longer inputs are truncated.

This PR introduces a FixedArrayLength decoding option

Thanks, I'll take a look after wrapping up in-progress work needed to release v2.9.5 and v2.10.0.

In the meantime, user-defined types implementing cbor.Marshaler can do this before the feature is added (not as convenient but available now).