feat: Support configuring pod and container security context

[jsirianni]

In an effort to support OpenShift, we need to make pod and container securityContext configurable by the user. This PR moves the hardcoded securityContext options to two new sections in the values file.

podSecurityContext:
  fsGroup: 65534

containerSecurityContext:
  runAsNonRoot: true
  runAsUser: 65534
  readOnlyRootFilesystem: true
  capabilities:
    drop: ["ALL"]

This change behaves identically by default, but allows user to modify securityContext.

Does not fully resolve OpenShift support, but this PR does address one of the first steps identified by a user here #16