extendSchema: preserve "description" and "extensions"

[igrlk]

👋

Closes #3629

Current behavior:

When calling extendSchema(schema, schemaModification), extended schema description is calculated as schemaDef?.description?.value, where

• schemaDef is a nullable schema definition, that will not be null only if schemaModification declares the type schema (it happens only when we call buildSchema or call extendSchema(schema, 'extend type __Schema { ... }'))
• schemaDef?.description is not undefined only when extendSchema is called from buildSchema, otherwise it's undefined because we don't allow a construction like:

"my awesome description" // error - graphql-js doesn't allow you to set the description when you extend schema extend type __Schema { ... } 

As a result, every extendSchema call besides the one in buildSchema will cause description to become undefined

Proposed solution:

Always fallback to the original schema description:
description: schemaDef?.description?.value ?? schemaConfig.description

It will allow us to make sure that we take a description from schemaDef when schema is being built and take original schema description in any other cases

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[IvanGoncharov]

Hi @igrlk
Thank you for this PR, I also noticed that we have the exact same problem with extensions so I fixed it in the same PR.

[igrlk]

Thank you, @IvanGoncharov!