Add serde support

[Neotamandua]

Summary

We wanna have serde support for the key types used by Moonlight and other bls types used in the execution-core events, so that we can add serde to execution-core event structures in a more concise way.

Possible solution design or implementation

Find out which datatypes in bls12_381-bls are used by event structs in execution-core and implement serde for them. We can also see if we want to add serde to other types as well, to avoid a dependency bump in the future when the need for additional serde functionality arises.