Graeme Coupar

feat!: rework extension handling on errors & responses

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#### Why are we making this change? GraphQl allows a server to provide arbitrary extensions on responses & errors within a response. Currently cynic only supports decoding these extensions on...

Need to support the changes from [the 2025 version of the spec](https://github.com/graphql/graphql-spec/blob/main/changelogs/September2025.md). This is a placeholder issue - I'll attach sub-issues when I get a chance.

fix: expose more of the queries::builders types.

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A lot of these are used by the derives and will be useful for generating dynamic queries, but don't appear in the documentation because they're not public enough.