Cyclic dependency between light-client and testgen crates

[adizere]

As part of testing the IBC relayer in ibc-rs, we would like to have the capability of generating light blocks. See, for instance, this issue informalsystems/hermes#158.

After discussing briefly with @andrey-kuprianov and @Shivani912, I understand that this is the current state of the light block generation testgen feature:

• light block generation feature was added (Adds light block generation to testgen #534) but unfortunately it also introduced a circular dependency between crates testgen and light-client;
• the feature was subsequently reverted (Fixes dependency loop #545), fixing the dependency loop;
• the cause for the dependency loop seems to be in the LightBlock struct, which testgen needs but it's defined in light-client (so the former crate would need to depend on the latter)

Possible solutions to avoiding the dependency loop:

• Option 1. Shivani has a branch Testgen: LightBlock generation without cyclic dependency #606 with the feature, and the temporary fix is to copy the LightBlock definition into testgen
• Option 2. Move the LightBlock outside of light-client crate and into some (new?) "common types" crate, so that both light-client and testgen would depend on this new crate.

Other solutions or thoughts?