Early init FuseboxClientMetadataModel

[motiz88]

Summary

Followup from #24. We want to not just positively detect the Fusebox client, but detect the use of non-Fusebox clients as well. To this end, we want to guarantee that the Fusebox frontend always sends FuseboxClient.setClientMetadata before (say) Runtime.enable, so that if we receive a Runtime.enable message without a preceding FuseboxClient.setClientMetadata, we know we're dealing with a non-Fusebox client.

As mentioned in a code comment, there's a proper way to express dependencies between frontend models, and we should do that. For now, I want to avoid duplicating the work that @hoxyq is currently doing in the stack of #19 to set up the first RN-specific models. We can follow up on this when those PRs have landed.

Test plan

Tested as part of https://www.internalfb.com/diff/D55122115.

Upstreaming plan

• This commit should be sent as a patch to the upstream devtools-frontend repo. I've reviewed the contribution guide.
• This commit is React Native-specific and cannot be upstreamed.