feat: support functional swr context config

[huozhi]

Background

SWRConfig now merges the config passed in value property with SWR config from parent context, but this bring a problem that users have to be aware of how many layers of context are existing and there's no way to ignore the config from parent context

Feature

• Support (parentConfig? Config) => Config overloading for SWRConfig value property

Then you can pick any properties from parent config and decide how to combine with the current SWR config, or you can just ignore any config property from parent context. e.g. you have a useSWR hook inside very deep level of the react tree, but you want it to keep the default config, you can just ignore all configs by ignoring all the parent configs.

Example

<SWRConfig value={config1}>
  <Page>
    <SWRConfig value={() => ({})}>
      <Main />
    </SWRConfig>
</SWRConfig>

[codesandbox-ci]

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Latest deployment of this branch, based on commit 1b254e4:

Sandbox	Source
SWR-Basic	Configuration
SWR-States	Configuration
SWR-Infinite	Configuration
SWR-SSR	Configuration

[shuding]

Looks great, thanks