add assumeImmutableResults typing to apollo boost PresetConfig interface

[bencoullie]

What

Fix PresetConfig interface (missing assumeImmutableResults property) in Apollo Boost.

Related Issue

#5570

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

[apollo-cla]

@bencoullie: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[hwillson]

Thanks @bencoullie!