[feature] #4060: add support for boxed slices in FFI

[mversic]

Description

Linked issue

Closes #4060

Benefits

Checklist

• I've read CONTRIBUTING.md
• I've used the standard signed-off commit format (or will squash just before merging)
• All applicable CI checks pass (or I promised to make them pass later)
• (optional) I've written unit tests for the code changes
• I replied to all comments after code review, marking all implemented changes with thumbs up

[coveralls]

Pull Request Test Coverage Report for Build 6942300741

• 34 of 94 (36.17%) changed or added relevant lines in 3 files are covered.
• 6810 unchanged lines in 127 files lost coverage.
• Overall coverage decreased (-3.1%) to 56.322%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ffi/src/repr_c.rs	24	84	28.57%

Files with Coverage Reduction	New Missed Lines	%
config/base/derive/src/view.rs	1	99.37%
config/src/block_sync.rs	1	95.0%
config/src/network.rs	1	93.75%
config/src/torii.rs	1	95.45%
config/src/wasm.rs	1	87.5%
core/src/smartcontracts/isi/block.rs	1	87.5%
config/src/kura.rs	2	79.41%
config/src/lib.rs	2	0.0%
ffi/src/option.rs	2	71.43%
config/src/genesis.rs	3	72.92%

Totals
Change from base Build 5423219773:	-3.1%
Covered Lines:	23002
Relevant Lines:	40840

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💛 - Coveralls