Add MIT or CC0-1.0 SPDX-License to interface

As discussed in #eip-editors, to clarify the interface is licensed
under CC0-1.0 (as the EIP) but also under MIT.

MIT ensures that any implementation of the interface is
guaranteed to be free of eventual patent- and license claims.

EIPS/eip-6956.md

@@ -95,6 +95,7 @@ The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "S
 Every contract compliant to this standard MUST implement the [the proposed standard interface](../assets/eip-6956/contracts/IERC6956.sol), [ERC-721](eip-721.md) and [ERC-165](eip-165.md) interfaces and is subject to [Caveats](#caveats-for-base-interface) below:
 
 ```solidity
+// SPDX-License-Identifier: MIT OR CC0-1.0
 pragma solidity ^0.8.18;
 
 /**
@@ -341,6 +342,7 @@ export async function minimalAttestationSample() {
 Every contract compliant to this standard MAY implement the [proposed AttestationLimited interface](../assets/eip-6956/contracts/IERC6956AttestationLimited.sol) and is subject to [Caveats](#caveats-for-attestationlimited-interface) below:
 
 ```solidity
+// SPDX-License-Identifier: MIT OR CC0-1.0
 pragma solidity ^0.8.18;
 import "./IERC6956.sol";
 
@@ -396,6 +398,7 @@ interface IERC6956AttestationLimited is IERC6956 {
 Every contract compliant to this extension MAY implement the proposed [Floatable interface](../assets/eip-6956/contracts/IERC6956Floatable.sol) and is subject to [Caveats](#caveats-for-floatable-interface) below:
 
 ```solidity
+// SPDX-License-Identifier: MIT OR CC0-1.0
 pragma solidity ^0.8.18;
 import "./IERC6956.sol";
 
@@ -449,6 +452,7 @@ If `floating(anchor)` returns true, the token identified by `tokenByAnchor(ancho
 Every contract compliant to this extension MAY implement the proposed [ValidAnchors interface](../assets/eip-6956/contracts/IERC6956ValidAnchors.sol) and is subject to [Caveats](#caveats-for-validanchors-interface) below:
 
 ```solidity
+// SPDX-License-Identifier: MIT OR CC0-1.0
 pragma solidity ^0.8.18;
 import "./IERC6956.sol";