In a previous PR (https://github.com/prometheus-community/helm-charts/pull/194)
support for specifying "extraVolumes" was introduced.
However, these extra volumes cannot be mounted in the main blackbox-exporter
container (only in sidecar containers).
This patch adds a new value "extraVolumeMounts" which lets the user mount
arbitrary volumes on the main container.
Signed-off-by: Jack Henschel <jackdev@mailbox.org>