[Enhancement] Enable custom be.conf overrides via Helm values (Doris Disaggregated)

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/doris-operator/issues?q=is%3Aissue) and found no similar issues.

### Description

Today the doris-disaggregated Helm chart generates a BE be.conf ConfigMap with fixed content, which makes it impossible to inject additional BE configuration from values.yaml.
This blocks users from tuning BE behavior without forking or patching the chart.
Additionally, the default type of msSpec.fdb.namespace is inconsistent, and nodeSelector placeholders are missing from values.yaml for FE/Compute.

### Solution

- Append user-provided BE config from computeSpec.beConf to the generated be.conf in the BE ConfigMap (without replacing the defaults).
- Add computeSpec.beConf to values.yaml with a commented example.
- Normalize the default for msSpec.fdb.namespace to an empty string ("") to avoid type issues.
- Add feSpec.nodeSelector and computeSpec.nodeSelector placeholders in values.yaml for easier customization.

### Are you willing to submit PR?

- [x] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)