Declare IPC marker fields in the schema

[mstange]

IPC markers currently don't have a schema, or if they do, it's a broken schema from an upgrader.

We need to add a schema and declare all the IPC marker fields in it, for #5302.

We can probably keep all the custom handling of IPC markers for now, i.e. any code which does things based on data.type and which expects certain fields to be present based on the type. We just need to make it so that the fields don't get discarded if we change how marker fields are stored in the processed format.

We currently add an IPC marker schema when computing the derived profile data in selectors. This schema describes the synthesized IPC markers we create when processing the raw markers. We should rename this schema to DerivedIPC so that it doesn't conflict with the new IPC schema that''ll be stored in the profile.

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