fix: Allow overwriting of rendered template in merlin and turing charts

[shydefoo]

Motivation

• Current implementation of rendering templates of merlin and turing using render field is incomplete. This MR introduces a new field overwrites, to overwrite specific fields that the render produces.

Modification

• New render.overwrites field to overwrite portions of the rendered template
• If render is empty, falls back to original mechanism of using .Values.config and $defaultConfig

For example, the current implementation does not allow the following config to be rendered in merlin:

      BaseImages:
        3.7.*:
          ImageName: ghcr.io/caraml-dev/merlin/merlin-pyfunc-base-py37:0.34.0-rc1
          DockerfilePath: "python/pyfunc-server/docker/Dockerfile"
          BuildContextURI: "git://github.com/caraml-dev/merlin.git#refs/tags/v0.34.0-rc1"
        ...
        # Test MLFlow upgrade
        3.10.*:
          ImageName: ghcr.io/caraml-dev/merlin/merlin-pyfunc-base-py310:0.0.0-f9fda74f934ebf546e4581495d93120e37f6a36b
          DockerfilePath: "python/pyfunc-server/docker/Dockerfile"
          BuildContextURI: "git://github.com/caraml-dev/merlin.git#refs/pull/465"

without setting rendered to an empty dictionary. This is because the existing config will be overwritten by the rendered template if rendered is set.

In order to get around this, a new field overwrites is added to the rendered dictionary, which allows users to specify which fields in the rendered template should be overwritten. The same can be applied to turing.

overwrites is like an escape hatch, it's not expected to be used except for testing purposes.
defaultConfig uses some values from .Values.config which makes it quite hard to use .Values.config to overwrite the rendered template sometimes, and have the rendered template overwrite other parts

Checklist

• Chart version bumped
• README.md updated