[extension/headerssetter] mask the default_value field to hide possible sensitive info

[dloucasfx]

Component(s)

extension/headerssetter

Describe the issue you're reporting

The default_value can hold sensitive values, especially if we are using it to set tokens.
Suggesting to use configopaque.String instead of string for default_value

Pushing a PR shortly

[github-actions]

Pinging code owners:

• extension/headerssetter: @VihasMakwana

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