[stable/rbac-manager] allow setting custom securityContext

[eyenx]

Why This PR?

As of now, no additional securityContext on the Pod Level can be set over the chart, this allows it and also enables us to configure the default SecurityContext on container level

Changes Changes proposed in this pull request:

• add securityContext value to set a SecurityContext on pod level
• add containerSecurityConatext value with the default set in the template to enable us to customize the SecurityContext on container level

Checklist:

• [x] I have included the name of the chart in the title of this PR in square brackets i.e. [stable/goldilocks].
• [x] I have updated the chart version in Chart.yaml following Semantic Versioning.
• [x] Any new values are backwards compatible and/or have sensible default.
• [x] Any new values have been added to the README for the Chart, or helm-docs --sort-values-order=file has been run for the charts that support it.

[CLAassistant]

All committers have signed the CLA.