editoast: `SearchPayload.object` should be an enum

[emersion]

Description and goal

Right now it's a String. This is not type-safe: the generated OpenAPI (and TypeScript code) will not complain when passing an invalid object type. This is especially risky when handling cache tags here:

osrd/front/src/common/api/osrdEditoastApi.ts

Line 352 in fbada52

providesTags: (_result, _error, args) =>

Turn SearchPayload.object into an enum with one entry per object type.

Acceptance criteria

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