Unspecified enum as zero value for InterchainAccountPacketData Type

[damiannolan]

Summary

The interchain accounts proto enum Type, used in InterchainAccountPacketData should contain an UNSPECIFIED/UNKNOWN enum as its zero value.
This can also be used for basic validation, ensuring an appropriate Type enum has been set.

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For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged/assigned

[gjermundgaraba]

This issue was mistakenly reopened during some integration work, so closing again now.