[opt](cloud) Support multiple config files in start.sh

[gavinchou]

What problem does this PR solve?

Support loading cloud start environment variables from multiple config files without forcing deployments to rely on process_name as the only config file name.

This keeps the default behavior of loading ${process_name}.conf, and adds support for CONF_FILES with comma-separated or whitespace-separated entries, glob patterns, and absolute paths. Config files are loaded in order, later files can override earlier files, while variables already set before config loading keep priority.

Issue Number: close #xxx

Related PR: #xxx

Problem Summary: cloud/script/start.sh previously loaded only ${process_name}.conf, which made multi-instance or layered config deployments harder to express.

Release note

Support specifying multiple cloud config files through CONF_FILES in cloud/script/start.sh.

Check List (For Author)

• Test: Manual test
  • Verified default ${process_name}.conf loading.
  • Verified comma-separated and whitespace-separated CONF_FILES lists.
  • Verified glob pattern loading.
  • Verified missing config files are skipped.
  • Verified absolute config file paths.
  • Verified pre-existing environment variables are not overridden by config files.
  • Ran bash -n cloud/script/start.sh.
• Behavior changed: Yes. cloud/script/start.sh can now load multiple config files through CONF_FILES.
• Does this need documentation: No

[hello-stephen]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[gavinchou]

run buildall

[gavinchou]

/review

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[gavinchou]

/review

[github-actions]

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