nvs: support StorageIterate

[codyps]

This implements StorageIterate for EspNvs. Note that while the Error on iteration is setup to work, the actual implementation in esp-idf does not return any errors during iteration (other than "Not Found", indicating iteration is complete). As a result, next() will always return Some(Ok(...)) or None and never Some(Err(...)).

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This depends on the esp-idf 5 iteration APIs (the ones in esp-idf 4 aren't supported) plus the change in espressif/esp-idf#11118. (Which we can apply in esp-idf-sys).

Plan on landing this:

1. restrict to esp-idf 5
2. have esp-idf-sys apply nvs: support iteration over namespace by index (IDFGH-9782) espressif/esp-idf#11118 to the esp-idf we build (5 & master)

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embedded-svc PR: esp-rs/embedded-svc#48

[ivmarkov]

See this though. Let me know what you think.