[Xray] Add ability to configure defaultRetentionDaysForIndexedRepo for Xray

[gregoryboue]

Is this a request for help?: No

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Is this a BUG REPORT or FEATURE REQUEST? (choose one): FEATURE REQUEST

Currently the Xray indexing default retention period can be defined in Xray System yaml with server.repo.defaultRetentionDaysForIndexedRepo settings.

If we want to configure this settings, we have to override xray system yaml (see systemYamlOverride), so it would be great to be able to configure this setting via a value in the chart's values.yaml file.