Add custom metadata storage

[vatsa287]

This is a draft implementation of having a custom metadata of MaxMetaPairLength in asset, but in general this can be replicated across the chain.
One of the current use-case would be to store created_at: UTC of old_chain_block after migration so apps can access this value instead of newly created block time.

How it is implemented?

• For demonstration have made this to asset specific.
• Create a storage named AssetMeta to store metadata of storage. It allows to custom k:v of MaxMetaPairLength currently set to 5, where the identifier of the asset is the first key, and second key is a custom type.
• When lookup'ed on using the identifier we get a list of all the k:v of the pallet metadata, but when used with a custom key it produces a result with the custom value.

Ex:

• Create a metapairs on the SDK, with string type k:v which has custom keys and values attached to the identifier of the asset.

    const metaPairs: [string, string][] = [
      ["key1", "value1"],
      ["key2", "value2"],
      ["key3", "value3"],
      ["key4", "value4"],
      ["key5", "value5"],
    ];

    const tx = api.tx.asset.issue(
      assetEntry.entry,
      assetEntry.digest,
      authorizationId,
      metaPairs
    )

• Chain-state after execution with all k:v seen,

• Chain-state after execution with a particular key query,

[vatsa287]

@amarts @smohan-dw @NiranjanAP @atreyasuraj ^^