fix: move db credentials into a secret

[adwk67]

Description

fixes #352

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

# Author
- [x] Changes are OpenShift compatible
- [x] CRD changes approved (implicitly as the same as https://github.com/stackabletech/hive-operator/pull/452)
- [x] CRD documentation for all fields, following the [style guide](https://docs.stackable.tech/home/nightly/contributor/docs-style-guide).
- [x] Helm chart can be installed and deployed operator works
- [x] Integration tests passed (for non trivial changes)
- [x] Changes need to be "offline" compatible

# Reviewer
- [x] Code contains useful comments
- [ ] Code contains useful logging statements
- [x] (Integration-)Test cases added
- [ ] Documentation added or updated. Follows the [style guide](https://docs.stackable.tech/home/nightly/contributor/docs-style-guide).
- [x] Changelog updated
- [x] Cargo.toml only contains references to git tags (not specific commits or branches)

# Acceptance
- [ ] Feature Tracker has been updated
- [ ] Proper release label has been added
- [ ] [Roadmap](https://github.com/orgs/stackabletech/projects/25/views/1) has been updated

[maltesander]

Nitpicking and a question, LGTM otherwise!

[adwk67]

Local tests:

--- PASS: kuttl (4735.62s)
    --- PASS: kuttl/harness (0.00s)
        --- PASS: kuttl/harness/oidc_druid-latest-28.0.1_zookeeper-latest-3.9.2_s3-use-tls-true_openshift-false (475.44s)
        --- PASS: kuttl/harness/cluster-operation_druid-latest-28.0.1_zookeeper-latest-3.9.2_hadoop-latest-3.3.6_openshift-false (643.17s)
        --- PASS: kuttl/harness/tls_druid-latest-28.0.1_zookeeper-latest-3.9.2_tls-mode-internal-and-server-tls_openshift-false (354.22s)
        --- PASS: kuttl/harness/ldap-authentication_druid-28.0.1_zookeeper-latest-3.9.2_opa-0.61.0_hadoop-latest-3.3.6_ldap-use-tls-true_ldap-no-bind-credentials-false_openshift-false (261.79s)
        --- PASS: kuttl/harness/authorizer_druid-28.0.1_zookeeper-latest-3.9.2_opa-0.61.0_hadoop-3.3.6_openshift-false (287.51s)
        --- PASS: kuttl/harness/ingestion-s3-ext_druid-latest-28.0.1_zookeeper-latest-3.9.2_hadoop-3.3.6_openshift-false (335.59s)
        --- PASS: kuttl/harness/logging_druid-28.0.1_zookeeper-latest-3.9.2_hadoop-3.3.6_openshift-false (319.82s)
        --- PASS: kuttl/harness/s3-deep-storage_druid-latest-28.0.1_zookeeper-latest-3.9.2_s3-use-tls-true_openshift-false (240.04s)
        --- PASS: kuttl/harness/tls_druid-latest-28.0.1_zookeeper-latest-3.9.2_tls-mode-no-tls_openshift-false (227.01s)
        --- PASS: kuttl/harness/orphaned-resources_druid-latest-28.0.1_zookeeper-latest-3.9.2_hadoop-3.3.6_openshift-false (338.17s)
        --- PASS: kuttl/harness/tls_druid-latest-28.0.1_zookeeper-latest-3.9.2_tls-mode-internal-and-server-tls-and-tls-client-auth_openshift-false (302.12s)
        --- PASS: kuttl/harness/ingestion-no-s3-ext_druid-latest-28.0.1_zookeeper-latest-3.9.2_hadoop-3.3.6_openshift-false (248.79s)
        --- PASS: kuttl/harness/smoke_druid-28.0.1_zookeeper-3.8.3_hadoop-3.3.6_openshift-false (284.56s)
        --- PASS: kuttl/harness/resources_druid-latest-28.0.1_zookeeper-latest-3.9.2_openshift-false (110.81s)
        --- PASS: kuttl/harness/hdfs-deep-storage_druid-latest-28.0.1_zookeeper-latest-3.9.2_hadoop-3.3.6_openshift-false (305.93s)

[adwk67]

Jenkins: 🟢 https://ci.stackable.tech/view/02%20Operator%20Tests%20(custom)/job/druid-operator-it-custom/136/

[maltesander]

LGTM!