Support for type normalization before encoding config (e.g., convert "true" string to true boolean)

[sanyam03]

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Problem Description

Describe the feature

When using viper.WriteConfig or a custom write implementation (via marshalWriter), values in the internal config map like "true" (string) are written to the output file (e.g., YAML) as "true" instead of true (boolean).

This usually happens when config values are updated or set programmatically from sources that store everything as strings (e.g., CLI flags, env vars, encrypted configs, etc).

Problem

This causes configuration types to be misleading in output YAML:

packager:
  example_project:
    trust: "true"   # should be: true (bool)


### Proposed Solution

A type normalization step before encoding, where "true" / "false" strings could be coerced into booleans (similarly for numeric strings).

A hook/callback mechanism (e.g., BeforeMarshal(func(map[string]any) map[string]any)) so users can implement normalization logic themselves before Viper marshals the config.

An option or flag like viper.NormalizeTypesOnWrite(true) that opts into this behavior.

### Alternatives Considered

_No response_

### Additional Information

_No response_

[github-actions]

Issues with no activity for 30 days are marked stale and subject to being closed.

[sagikazarmark]

This is a good feature to add and probably not that hard to implement.