Add a writeQuery method to replace local mutations

[sedenko93]

Hi All,

We are building new applications using apollo-angular and apollo-client as state management. Recently we updated
from v2 to v3. Our state management is driven by Queries & Mutations using local resolvers. We are building queries & mutations with the @client directive and generate angular services using codegen, this has worked great so far.

When migrating to v3 i've seen apollo-client has deprecated local resolvers causing this way of "mutating" our state to be replaced by a cache.writeQuery method and using the typePolicies to merge the data, but the writeQuery method isn't supplied by the generated services that extend the apollo-angular Query class.

I feel we miss a step here, i would expect the apollo-angular to supply a writeQuery for a query and simply replace local mutations by writeQueries. Is this right or am i miss understanding how the flow is supposed to be now?

I would like to contribute and extend the Query class, this package is very important to us and our success of writing
our new applications. @kamilkisiela thanks for your great work so far 👏🏼, what are your thoughts on this?