Add support for specifying TLS auth data via `.spec.certSecretRef` in
HelmRepository and log a deprecation warning if TLS is configured via
`.spec.secretRef`. Introduce (and refactor) Helm client builder and
auth helpers to reduce duplicated code and increase uniformity and
testability.
Signed-off-by: Sanskar Jaiswal <jaiswalsanskar078@gmail.com>