[bitnami/thanos] Add support for image digest apart from tag

[carrodher]

Description of the change

From now on it will be possible to set an image.digest instead of image.tag to pull the container image. Please note it is needed to release a bitnami/common version with those changes (see #11830).

Once applied the changes, this is the result when setting the image.digest parameter in the values.yaml:

## Bitnami etcd image version
## ref: https://hub.docker.com/r/bitnami/etcd/tags/
## @param image.registry etcd image registry
## @param image.repository etcd image name
## @param image.tag etcd image tag
## @param image.digest etcd image digest in the way sha256:aa.... Please note this parameter, if set, will override the tag
##
image:    
  registry: docker.io
  repository: bitnami/etcd
  tag: 3.5.4-debian-11-r22
  digest: sha256:507bc997779af5f0419ad917167ba1c9a2ceb936f2c1e82fb0f687e6c1296897

$ helm template . -s templates/statefulset.yaml | grep 'image:'
          image: docker.io/bitnami/etcd@sha256:507bc997779af5f0419ad917167ba1c9a2ceb936f2c1e82fb0f687e6c1296897

In the same way, when the image.digest is empty (default value), this is the result:

## Bitnami etcd image version
## ref: https://hub.docker.com/r/bitnami/etcd/tags/
## @param image.registry etcd image registry
## @param image.repository etcd image name
## @param image.tag etcd image tag
## @param image.digest etcd image digest in the way sha256:aa.... Please note this parameter, if set, will override the tag
##
image:    
  registry: docker.io
  repository: bitnami/etcd
  tag: 3.5.4-debian-11-r22
  digest: ""

$ helm template . -s templates/statefulset.yaml | grep 'image:'
          image: docker.io/bitnami/etcd:3.5.4-debian-11-r22

[rafariossaa]

LGTM