[FEATURE] Add support for emptyDir file storage

[rickardsjp]

TLDR: Adds support for ephemeral emptyDir storage as an alternative to PersistentVolumeClaims when using file-based Perses databases.

Motivation

StatefulSets with PVCs aren't always necessary for Perses. Since the operator creates projects, datasources, and dashboards via HTTP requests on pod startup, ephemeral storage works well for e.g. read-only instances. This eliminates provisioning persistent disks or configuring external databases for such use cases.

This approach follows the Prometheus Operator pattern, which even defaults to emptyDir storage when no storage configuration is specified.

Usage

spec:
  config:
    database:
      file:
        folder: "/perses"
        extension: "yaml"
  storage:
    useEmptyDir: true
    emptyDirSizeLimit: 1Gi  # optional

Creates a Deployment with ephemeral storage. Data is lost on pod restart. Note that the storageClass and size fields are ignored when setting useEmptyDir: true, which also follows the example set by the Prometheus operator.

WDYT?