Implemented burn authz handler (#663)

# Description
This PR intoruduces authz handler to grant burn athorization with quota.
# Reviewers checklist:
- [ ] Try to write more meaningful comments with clear actions to be taken.
- [ ] Nit-picking should be unblocking. Focus on core issues.

# Authors checklist
- [x] Provide a concise and meaningful description
- [x] Review the code yourself first, before making the PR.
- [x] Annotate your PR in places that require explanation.
- [x] Think and try to split the PR to smaller PR if it is big.

docs/api.md

@@ -6,6 +6,7 @@
 ## Table of Contents
 
 - [coreum/asset/ft/v1/authz.proto](#coreum/asset/ft/v1/authz.proto)
+    - [BurnAuthorization](#coreum.asset.ft.v1.BurnAuthorization)
     - [MintAuthorization](#coreum.asset.ft.v1.MintAuthorization)
 
 - [coreum/asset/ft/v1/event.proto](#coreum/asset/ft/v1/event.proto)
@@ -329,6 +330,22 @@
 
 
 
+<a name="coreum.asset.ft.v1.BurnAuthorization"></a>
+
+### BurnAuthorization
+BurnAuthorization allows the grantee to burn up to burn_limit coin from
+the granter's account.
+
+
+| Field | Type | Label | Description |
+| ----- | ---- | ----- | ----------- |
+| `burn_limit` | [cosmos.base.v1beta1.Coin](#cosmos.base.v1beta1.Coin) | repeated |  |
+
+
+
+
+
+
 <a name="coreum.asset.ft.v1.MintAuthorization"></a>
 
 ### MintAuthorization
@@ -338,7 +355,7 @@ the granter's account.
 
 | Field | Type | Label | Description |
 | ----- | ---- | ----- | ----------- |
-| `mint_limit` | [cosmos.base.v1beta1.Coin](#cosmos.base.v1beta1.Coin) |  |  |
+| `mint_limit` | [cosmos.base.v1beta1.Coin](#cosmos.base.v1beta1.Coin) | repeated |  |