Custom serde serialization for DefaultPublicKey

[bkolad]

Description

This PR introduces custom serde serialization and deserialization for the DefaultPublicKey type. This means that the public key can now be automatically serialized as hexadecimal without requiring manual conversion to DefaultPublicKeyHex

Testing

Unit test added.

Docs

No changes.

[codecov]

Codecov Report

Merging #962 (6f43dc0) into nightly (f31359e) will increase coverage by 0.1%.
The diff coverage is 92.5%.

❗ Current head 6f43dc0 differs from pull request most recent head e476fd5. Consider uploading reports for the commit e476fd5 to get more accurate results

Files	Coverage Δ
...module-implementations/sov-accounts/src/genesis.rs	95.0% <100.0%> (-0.3%)	⬇️
...m/module-implementations/sov-accounts/src/hooks.rs	96.2% <100.0%> (ø)
...m/module-implementations/sov-accounts/src/tests.rs	99.5% <100.0%> (ø)
...le-system/sov-modules-api/src/default_signature.rs	76.2% <100.0%> (ø)
module-system/sov-modules-api/src/lib.rs	91.3% <ø> (ø)
module-system/sov-modules-api/src/pub_key_hex.rs	93.3% <100.0%> (+5.3%)	⬆️
...tem/module-implementations/sov-accounts/src/lib.rs	72.7% <0.0%> (ø)
module-system/sov-modules-api/src/serde_pub_key.rs	91.4% <91.4%> (ø)

... and 2 files with indirect coverage changes

[neysofu]

LGTM, I have just added a comment about AccountConfig generics. Feel free to either address or ignore and merge.