[ICS02] Use Vec<u8> for HeaderAttribute instead of Any

[Farhad-Shabani]

Problem Statement

Context: HeaderAttribute is a field of our UpdateClient event.

• Taking ibc-go as a reference implementation and reviewing the suggested spec for events here, it appears that the header attribute was never intended to be of type Any, but rather to consist of hex-encoded raw Protobuf bytes.

• Looking at Hermes too, this header is not used for parsing into a concrete header type anywhere.

• This issue originated from Compile errors of v0.41.0 while only serde feature enabled #741, where enabling serde on HeaderAttribute with the Any type leads to a compilation error.