Make NVS namespace adjustable

[vodka-bears]

• Introduce static string nimble_nvs_namespace that is used instead of the old NIMBLE_NVS_NAMESPACE macro.
• Add function int ble_store_set_namespace(const char * namespace_str) that allows for changing namespace string.
• Rename NIMBLE_NVS_NAMESPACE to NIMBLE_NVS_NAMESPACE_DEFAULT.
• Replace occurrences of macro NIMBLE_NVS_NAMESPACE with variable nimble_nvs_namespace.

Description

Developing a custom project I realized that I need to run several devices with different addresses on the same hardware. The purpose of the change is to make it possible.

I abstained from mentioning the new function in any headers. Users can make a forward declaration if they need to change the namespace string.

Related

Fixes #95

Testing

Works well in my project. Bonds separately to the same host or to multiple hosts and bonding remains consistent and isolated.

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Checklist

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• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
• Git history is clean — commits are squashed to the minimum necessary.