railjson: add is_station field for operational points

[emersion]

A generic flag indicating whether an OP is a station will allow us to drop SNCF-specific logic.

This is part of #6146

Follow-up work:

• osrd-data
• Wire up search form
• Grep for "BV"/"00", switch to is_station

[emersion]

"stations" include freight, whereas "BV" is passenger specific. Was that considered?

Does "00" include freight as well? Is there a different secondary code for freight?

From a UX PoV, do we want to include or exclude freight stations?

(My understanding is: yes, no, yes, respectively.)

[emersion]

@Khoyo hasn't been addressed yet.

• If we go for a "station" field, I think we should include freight stations too. We don't have a good way to recognize these at SNCF, but it's fine to keep this best-effort and only populate the "station" field for a subset of stations (BV/00).
• If we want to specifically exclude freight stations from the special treatments (prioritization in UIs, default choice when user hasn't specified anything more specific), then we should rename the field to "passenger station".

Data question: do freight stations share the same trigram as passenger stations at SNCF?