Add wallet features

[tmm360]

Swarm extension can become a Swarm oriented wallet. This would complete implementation of ethersphere/bee#2905 with the client side.

We need a way for upload to a node chunks that was already signed by the client with postage stamps.
This primarily permits to publish contents signed with the user's address instead of the node's one.

This is useful in all cases where a node is used by more than one user. Currently every published content is signed by node, and so the node owner is the first responsible of uploaded contents, where instead the final user should be.

Moreover, this permits to discharge node from computing load of sign every chunk, distributing it to clients.

This issue requests to implement an Ethereum wallet integrated, and features for directly purchase postage batches. These can be used for sign contents, and the wallet must be able to upload them on Bee node.