Adds Inventory Notary Proofs and Bridge Handlers

[Step7750]

• Adds ability to fetch Steam inventory via api.steampowered.com on the bridge
• Adds ability to notarize a Steam inventory request

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Note

Medium Risk
Adds a new bridge endpoint that makes authenticated calls to api.steampowered.com, plus expands notary proof surface area; main risk is incorrect parameterization or unexpected Steam API responses affecting proofing/fetch flows.

Overview
Adds a new bridge handler, FetchSteamPoweredInventory, that uses a user access token to call Steam’s IEconService/GetInventoryItemsWithDescriptions API and return assets/descriptions (plus optional properties/pagination).

Extends the request routing/types to support a new RequestType.FETCH_STEAM_POWERED_INVENTORY, and adds ProofType.INVENTORY so notary proofs can generate/validate inventory request URLs against the SteamPowered inventory endpoint.

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[GODrums]

Tested with items in the 10d period and works as intended 👍