[iOS] Refactor Page Context State Into Computation And Attachment Enums

This change refactors the BWGPageContextState enum by splitting it
into two more specific enums: BWGPageContextComputationState and
BWGPageContextAttachmentState. This allows for a more granular
representation of the page context's status.

The new enums are mirrored with corresponding GCRGemini versions,
and conversion functions have been added to translate between the
BWG and GCRGemini types.

The original BWGPageContextState and its conversion function are
preserved to avoid breaking existing dependencies.

CL Chain:
    crrev.com/i/8469317
--> crrev.com/c/6806161

Bug: 434662294
Change-Id: Iad5274521104e17b047bc91c9f138dd976ebb725
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6806161
Commit-Queue: Joemer Ramos <joemerramos@google.com>
Reviewed-by: Nicolas MacBeth <nicolasmacbeth@google.com>
Reviewed-by: Adam Arcaro <adamta@google.com>
Cr-Commit-Position: refs/heads/main@{#1495023}