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 perform many similar actions.
+For example - `minting`, `burning` and `transferring`.
+Each app may have their own way of performing these actions.
+This introduces inconsistency in capturing these events.
+This extension addresses that.
+
+It's common for NFT apps to transfer many or one token at a time.
+Other applications need to track these and similar events consistently.
+If not, tracking state across many NFT-driven apps become infeasible.
+
+We need a standard way to capture events.
+This was discussed here https://github.com/near/NEPs/issues/254.
+
+## Interface
+
+Many apps use different interfaces that represent the same action.
+This interface standardizes that process.
+It captures these actions 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`
+// * `memo`: optional message
+// * `data`: associate event data
+interface EventLogData {
+    standard:string,
+    version:string,
+    event:string,
+    data: NftMintLog[]|NftTransferLog[]|NftBurnLog[]
+}
+
+// An event log to capture token minting
+// Arguments
+// * `owner_id`: "account.near"
+// * `token_id`: "1"
+// * `memo`: optional message
+interface NftMintLog {
+    owner_id:string,
+    token_ids:string[],
+    memo?:string
+}
+
+// An event log to capture token burning
+// Arguments
+// * `owner_id`: owner of tokens to burn
+// * `delegatee_id`: approved account to burn
+// * `token_ids`: ["1","2"]
+// * `memo`: optional message
+interface NftBurnLog {
+    owner_id:string,
+    delegatee_id:string,
+    token_ids:string[],
+    memo?:string
+}
+
+// An event log to capture token transfer
+// Arguments
+// * `delegatee_id`: approved account to burn
+// * `sender_id`: "account.near"
+// * `receiver_id`: "receiver.near"
+// * `token_id`: "12345abc"
+// * `memo`: optional message
+interface NftTransferLog {
+    delegatee_id:string,
+    sender_id:string,
+    receiver_id:string,
+    token_ids:string[],
+    memo?:string
+}
+```
+
+## Further methods
+
+Note that the example events covered above cover two different kinds of events:
+1. Events that are not specified in the NFT Standard (`nft_mint`, `nft_burn`)
+2. An event that is covered in the [NFT Core Standard](https://nomicon.io/Standards/NonFungibleToken/Core.html#nft-interface). (`nft_transfer`)
+
+This event standard also applies beyond the three events highlighted here, where future events follow the same convention of as the second type. For instance, if an NFT contract uses the [approval management standard](https://nomicon.io/Standards/NonFungibleToken/ApprovalManagement.html), it may emit an event for `nft_approve` if that's deemed as important by the developer community.
+
+Please feel free to open pull requests for extending the events standard detailed here as needs arise.
+
+## Drawbacks
+
+There is a known limitation of 16kb strings when capturing logs.
+This can be observed if `token_ids` vary in length across different apps.
+This impacts the amount of events that can be processed.