Add custom metadata and parameters for asset registry

[ebma]

We want to store a fee_per_second attribute for each asset defined in our asset registry. For this, we want to use the CustomMetadata of the asset registry. At the moment, we are using the CustomMetadata struct defined in the spacewalk primitives but it makes more sense to now define this struct in our runtimes instead.

TODO

• Create a CustomMetadata struct that replaces the one in asset_registry
• Add a fee_per_second parameter to it
• Add parameters for price oracle keys. We should create a new struct DiaKeys { blockchain: Vec<u8>, symbol: Vec<u8> } that allows using this info to create price info from our DIA oracle pallet.

[annatekl]

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