client: Add Keycloak authentication in explorer

It enables to login with Keycloak client-side and set access token in authorization header in order to perform authenticated calls to the GraphQL API.

Depends on swh-auth!53 (merged)

Some puppet config for Keycloak must be updated in order to use the feature with our Keycloak instance for production and staging.

It can also be tested in docker once swh-environment!259 (merged) is merged.

Related to #4652