Add readinessProbe to helm values

[raphaelzoellner]

Description

Adds a readinessProbe value to the helm chart such that the promscale-connector readinessProbe can be customized.
This allows to specify authentication for the readinessProbe.

E.g.:

values.yaml

readinessProbe:
  httpGet: null
  exec:
    command:
    - sh
    - -c
    - |-
      status_code=$(wget --server-response "http://${PROMSCALE_WEB_AUTH_USERNAME}:${PROMSCALE_WEB_AUTH_PASSWORD}@localhost:9201/healthz" 2>&1 | awk '/^  HTTP/{print $2}');
      if [[ "${status_code}" == "200" ]]; then exit 0; else exit 1; fi