Remove spruceid/siwe in favor of viem's implementation

[hiendaovinh]

What is the bug?
To use AuthKit, we have to polyfill Buffer in order to workaround this problem.
Cannot read properties of undefined (reading 'isBuffer')

While this could be handled with little effort of polyfill, I suggest that we move to a better implementation with https://viem.sh/docs/siwe/utilities/createSiweMessage.
This will result in less dependencies, less polyfill and better ts support.

Will happy to craft a pull request if needed.

[hiendaovinh]

We could take one more step and use viem instead of ethers for further reducing of dependencies.