[Client] Create hooks to fetch data for coverage summary

[nicholas-codecov]

We need to fetch some basic information to generate the dropdown summary string. We can grab the missesCount and partialsCount fields from the patchTotals object on the Comparison type in GraphQL. From this hook we will want to return the sum of the two values or return 0 if there is no missing lines.

Designs can be found here.

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Tasks

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1. Create hook to fetch coverage summary data for a given commit
2. Create hook to fetch coverage summary data for a given PR