Add connected wallets endpoint

[rickyrombo]

Thoughts:

• Testing the query and the endpoint seems fairly redundant in this case, but maybe the interesting thing is to test that the json keys are the correct and that the status code is correct
• Was tempted to switch over to array_agg and do the grouping in the query, but had to map the chain values anyways so seemed moot. In v2, we should just return the list. I don't think the output formatting by chain is very useful or relevant
• Making fixtures is somewhat nice, but I worry a bit about it discouraging us from setting up DB constraints to protect our data integrity?
• Not sure FullConnectedWallets made sense to make. Maybe better to just inline the mapping to the api? (There's no Full/non-full version of this)

[raymondjacobson]

lgtm - i think that we should have full and non full for everything. if the response shape is the exact same, that's ok

[stereosteve]

Making fixtures is somewhat nice, but I worry a bit about it discouraging us from setting up DB constraints to protect our data integrity?

I think we can set up meaningful constraints and still use fixtures...
like the blocks constraint is annoying and also worthless.
Adding user_id constraints would make sense and also the fixtures should satisfy those constraints anyway.