feat: bring boilerplate Wallet class in core library

[denbite]

A lot of projects that use wallet-selector are forced to put in their project boilerplate code of Wallet class over and over again

https://github.com/phoenixpulsar/auto-chain-verify/blob/master/src/wallets/near.js
https://github.com/aarav1656/near-ai-agent/blob/master/src/app/utils/near-wallet.js
https://github.com/finowl-near/finowl-app/blob/main/frontend/src/app/Wallets/near.js
https://github.com/partagexyz/fans-club/blob/main/src/wallets/near.js
https://github.com/NEARBuilders/devhub-rebuild-with-requests/blob/main/src/wallets/near-wallet.ts
https://github.com/LewinJun/near-demo/blob/0bc0e73ede2175a9b8ed964f66848281d2bb0ca2/my-books/src/wallets/near.js
and many more

This PR addresses the problem by implementing Wallet class as part of wallet-selector-core library. The class is written to be flexible enough for different use cases.

[gagdiez]

While I like the idea of bringing all the commonly used interface, I think it would be nicer if we could get it directly implemented in the Wallet, so people does not need to create a new object to use these methods

By implementing it in the wallets we should be able to also use the provider that the user configured on the selector, instead to instantiate a new one