feat(street-level): provider exports and a Mapillary Graph endpoint for hosts (#1324)

* feat(street-level): export the KartaView and Panoramax providers

A host that composes Street Level with its own provider list can already
import the Mapillary and Google Street View providers; KartaView and Panoramax
were reachable only through the default list, which needs a Mapillary source.
Exporting them lets a host offer the keyless providers without Mapillary.

* chore: changelog entry for the Street Level provider exports

* feat(street-level): send Mapillary Graph lookups to a source endpoint

A Mapillary source can take a graphEndpoint, a Graph API base that adds
the token itself. Its sequence lookups go there through the source's
transport, and the photo viewer uses MapillaryJS's stock GraphDataProvider
at that endpoint, so a host that proxies Mapillary needs no client token in
the page. Without it, behaviour is unchanged: direct Graph API calls with
the client token.

changelog.d/1324-street-level-provider-exports.md

@@ -0,0 +1,8 @@
+- Export the KartaView and Panoramax Street Level providers, so a host composing
+  its own provider list can offer the keyless providers without Mapillary
+  (samehkhamis, #1324).
+- A Mapillary source can take a `graphEndpoint`, a Graph API base that adds the
+  token itself (such as a same-origin proxy). Its sequence lookups go there
+  through the source's transport, and the photo viewer uses MapillaryJS's
+  Graph provider at that endpoint, so no client token is needed in the page
+  (samehkhamis, #1324).