Improve typings on Observable types

[ismail-syed]

The quilt TypeScript upgrade in #962 required some explicit return types on Observables. I started work on improving the explicit return types but that needed further changes. It was best to leave it out of the already large PR and should be addressed in the future PR. - context

Related packages:

• ./packages/jest-mock-apollo/dist/src/MockApolloLink.d.ts
• ./packages/jest-mock-apollo/src/MockApolloLink.ts
• ./packages/graphql-persisted/dist/src/tests/utilities.d.ts
• ./packages/graphql-persisted/src/tests/utilities.ts
• ./packages/graphql-testing/dist/src/links/mocks.d.ts
• ./packages/graphql-testing/src/links/mocks.ts

[stale]

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