feat: add NFT mint/transfer/burn event standard

specs/Standards/NonFungibleToken/Event.md

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+# Non-Fungible Token Event
+
+Version `1.0.0`
+
+## Summary
+
+Standard interfaces for NFT contract actions.
+
+## Motivation
+
+Nft driven apps such as marketplaces regularly perform many
+of the same acions such as `minting`, `burning` and `transfering`.
+Each app have their own way of performing these actions and it
+is difficult to consistently capture these events.
+This extension addresses that.
+
+For example, it's common in NFT marketplaces to have
+different methods of transfer a single token vs many 
+tokens. Other applications like wallets, will have
+a difficult time tracking consistently this information for many
+markets. This makes interactive with many nft driven apps 
+infeasible.
+
+Due to discussions here 
+https://github.com/near/NEPs/issues/254,
+a standard way to capture these events are needed.
+
+## Interface
+
+Many apps may use different mechanisms to perform their batching
+requirements. This interface standardises this process by capturing
+events of these activities through logs.
+
+```ts
+// Interface to capture an event
+// and return formatted event string.
+interface EventJsonLog {
+    // Takes `EventLogData` and returns
+    // `EVENT_JSON: <EVENT_LOG_DATA>`
+    (e:EventLogData):string
+}
+
+// Interface to capture data 
+// about an event
+// Arguments
+// * `standard`: name of standard e.g. nep171
+// * `version`: e.g. 1.0.0
+// * `event`: `nft_mint` | `nft_burn` | `nft_transfer`
+// * `data`: associate event data
+interface EventLogData {
+    standard:string,
+    version:string,
+    event:string,
+    data: NftMintLog[]|NftTransferLog[]|NftBurnLog
+}
+
+// An event log to capture token minting
+// Arguments
+// * `account_id`: "account.near"
+// * `token_id`: "1"
+interface NftMintLog {
+    owner_id:string,
+    token_id:string
+}
+
+// An event log to capture token burning
+// Arguments
+// * `owner_id`: owner of tokens to burn
+// * `token_ids`: ["1","2"]
+interface NftBurnLog {
+    owner_id:string,
+    token_ids:string[]
+}
+
+// An event log to capture token transfer
+// Arguments
+// * `sender_id`: "account.near"
+// * `receiver_id`: "receiver.near"
+// * `token_id`: "12345abc"
+interface NftTransferLog {
+    sender_id:string,
+    receiver_id:string,
+    token_id:string
+}
+```
+
+## Drawbacks
+
+There is a known limitation of 16kb strings when capturing logs.
+This can be observed from `token_ids` that may vary in length
+for different apps so the amount of logs that can
+be executed may vary.