refactor: reintroduce contentful sys commen resource attribute

[axe312ger]

This is part of #30855 and introduces the following changes:

• by moving metadata into Contentful's sys attribute we align our structure further into direction of Contentfuls GraphQL API Sys type

One side effect of this change is that we later can remove the Contentful SDK link resolution. This will lead to performance improvements and code simplification (https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-source-contentful/src/gatsby-node.js#L446-L492)

(Breaking) Changes

Metadata / Data from sys

Old Path	New Path	Comment
node_locale	sys.locale	this does not exists in Contentful's sys but we need it (for now) as Contentful GraphQL handles locales different
contentful_id	sys.id
spaceId	sys.spaceId
createdAt	sys.firstPublishedAt
updatedAt	sys.publishedAt
revision	sys.publishedVersion
new	sys.environmentId
sys.type	sys.contentType (and is now a real reference)

Sorting

allContentfulBoolean(sort: { fields: contentful_id }) {}

becomes

allContentfulBoolean(sort: { fields: sys___id }) {}

Filtering

contentfulDate(contentful_id: { eq: "38akBjGb3T1t4AjB87wQjo" }) {}

becomes

contentfulDate(sys: { id: { eq: "38akBjGb3T1t4AjB87wQjo" } }) {}

Rich Text fields

1. you can now query all Contentful content types in the references subfield, not matter if they are linked or not
2. the entry id can now be queried directly as child-field of references instead of fields within each referenced content type

JSON fields

Spaces within keys of JSON fields are now kept.

<p data-cy-value-born-at>Born at: {actor.Born_At}</p>

becomes

<p data-cy-value-born-at>Born at: {actor["Born At"]}</p>

[axe312ger]

This includes code from #31008 and is blocked thill the other one is merged & this one rebased