ERC-6909 interface (#664)

Closes #655 adding ERC 6909 interface

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Co-authored-by: zerosnacks <95942363+zerosnacks@users.noreply.github.com>

src/interfaces/IERC6909.sol

@@ -0,0 +1,72 @@
+// SPDX-License-Identifier: MIT
+pragma solidity >=0.6.2;
+
+import {IERC165} from "./IERC165.sol";
+
+/// @dev Required interface of an ERC-6909 compliant contract, as defined in
+/// https://eips.ethereum.org/EIPS/eip-6909
+interface IERC6909 is IERC165 {
+    /// @dev Emitted when the allowance of a `spender` for an `owner` is set for a token of type `id`.
+    event Approval(address indexed owner, address indexed spender, uint256 indexed id, uint256 amount);
+
+    /// @dev Emitted when `owner` grants or revokes operator status for a `spender`.
+    event OperatorSet(address indexed owner, address indexed spender, bool approved);
+
+    /// @dev Emitted when `amount` tokens of type `id` are moved from `sender` to `receiver` initiated by `caller`.
+    event Transfer(
+        address caller, address indexed sender, address indexed receiver, uint256 indexed id, uint256 amount
+    );
+
+    ///@dev Returns the amount of tokens of type `id` owned by `owner`.
+    function balanceOf(address owner, uint256 id) external view returns (uint256);
+
+    /// @dev Returns the amount of tokens of type `id` that `spender` is allowed to spend on behalf of `owner`.
+    /// NOTE: Does not include operator allowances.
+    function allowance(address owner, address spender, uint256 id) external view returns (uint256);
+
+    /// @dev Returns true if `spender` is set as an operator for `owner`.
+    function isOperator(address owner, address spender) external view returns (bool);
+
+    /// @dev Sets an approval to `spender` for `amount` tokens of type `id` from the caller's tokens.
+    /// Must return true.
+    function approve(address spender, uint256 id, uint256 amount) external returns (bool);
+
+    /// @dev Grants or revokes unlimited transfer permission of any token id to `spender` for the caller's tokens.
+    /// Must return true.
+    function setOperator(address spender, bool approved) external returns (bool);
+
+    /// @dev Transfers `amount` of token type `id` from the caller's account to `receiver`.
+    /// Must return true.
+    function transfer(address receiver, uint256 id, uint256 amount) external returns (bool);
+
+    /// @dev Transfers `amount` of token type `id` from `sender` to `receiver`.
+    /// Must return true.
+    function transferFrom(address sender, address receiver, uint256 id, uint256 amount) external returns (bool);
+}
+
+/// @dev Optional extension of {IERC6909} that adds metadata functions.
+interface IERC6909Metadata is IERC6909 {
+    /// @dev Returns the name of the token of type `id`.
+    function name(uint256 id) external view returns (string memory);
+
+    /// @dev Returns the ticker symbol of the token of type `id`.
+    function symbol(uint256 id) external view returns (string memory);
+
+    /// @dev Returns the number of decimals for the token of type `id`.
+    function decimals(uint256 id) external view returns (uint8);
+}
+
+/// @dev Optional extension of {IERC6909} that adds content URI functions.
+interface IERC6909ContentURI is IERC6909 {
+    /// @dev Returns URI for the contract.
+    function contractURI() external view returns (string memory);
+
+    /// @dev Returns the URI for the token of type `id`.
+    function tokenURI(uint256 id) external view returns (string memory);
+}
+
+/// @dev Optional extension of {IERC6909} that adds a token supply function.
+interface IERC6909TokenSupply is IERC6909 {
+    /// @dev Returns the total supply of the token of type `id`.
+    function totalSupply(uint256 id) external view returns (uint256);
+}