rpm: provision a place to cache data.

[frzifus]

Does it make sense to provision some space for the send queue?

Context: containers/ai-lab-recipes#666 (comment)

[Service]
# User and Group already exist.
# User=observability
# Group=observability
ExecStartPre=/bin/mkdir -p /var/cache/opentelemetry-collector
ExecStartPre=/bin/chown observability:observability /var/cache/opentelemetry-collector
...

cc @sallyom

[frzifus]

FTR, today we agreed to provide this default space and a filestorage default config.

cc @F-X64 @ConorC117