Expose `Attributes` type definition in public place

See https://github.com/getsentry/snuba/pull/8245#discussion_r3687009587.
It seems that users currently have to import with `from sentry_sdk._types import Attributes`

Start by inspecting sentry_sdk._types and the discussion linked in the issue to understand the current Attributes definition and the intended public import location. Expose the type through a public SDK path so users do not need to import from the private module, then verify that the public import works.

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