[feat] Flexible security contexts

[jjlgao]

Allows users to fully define the security contexts of their pods rather than only supporting a limited list. Previous format must be maintained for backwards compatibility, which necessitates the complex set of if statements.

This is meant to address these issues:

• Broader support for security context #124
• Chart does not respect securityContext settings #57

Tested locally, confirmed the following:

• helm template charts/retool --values charts/retool/ci/test-install-values.yaml runs successfully without error
• When securityContext.enabled=false, no security context block is added
• When securityContext.values={}, the legacy values are used
• When there are values in securityContext.values, those values are overridden, ignoring the legacy values

Update: changed PR to match telemetry deployment model of maintaining legacy params while providing the option to add extra custom ones.