Add new 'GraphQLSchema.getField' method

[IvanGoncharov]

Motivation: generalize it and remove code dublication in two places and
also allow to use as public API outside of graphql-js

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[IvanGoncharov]

cc @yaacovCR

[github-actions]

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[github-actions]

@github-actions run-benchmark

@IvanGoncharov Please, see benchmark results here: https://github.com/graphql/graphql-js/runs/6598090837?check_suite_focus=true#step:6:1

[IvanGoncharov]

@IvanGoncharov Please, see benchmark results here: graphql/graphql-js/runs/6598090837?check_suite_focus=true#step:6:1

Sadly, I need to figure out a way to run benchmark reliably on CI after #3604
But here is data from my machine where HEAD is this PR.

So performance slightly increases by ~1% due to using a switch instead of multiple if.

[yaacovCR]

Because this PR contains two changes ie change to switch and addition of unnecessary check for unions during execution, the benchmark results are not so accurate.

i have suggested adding two helpers. See #3519 (comment)

getObjectField and getCompositeField maybe?

[yaacovCR]

Sadly, I need to figure out a way to run benchmark reliably on CI after #3604

So it seems like #3604 broke benchmarking? Can we revert at least until fixed? Can we consider a 3rd party benchmarking solution as an additional dev-dependency so that we can outsource some of the benchmarking work?