Add fuzz tests to `pkg/scale`

[qdm12]

Task summary

#2629 was found randomly by adding a bad test case when writing tests for the trie proofs; this should not be found randomly and we should make use of fuzz tests instead. This is especially more important since we export it as pkg/scale, so other projects would be potentially affected.

• Add fuzz tests to check for panics for the decoder
• Add fuzz tests for the encoder + decoder
• Add fuzz tests to CI workflow