Debounce Username and E-mail check API calls

[Pessina]

Currently, the API calls to check if the username and email exist are triggered every time the user types a letter in the inputs.

This is not ideal as it consumes unnecessary resources. Therefore, the idea is to only trigger the checks after the user stops typing, thereby maintaining a good user experience and saving on resources.

Link to file: https://github.com/near/fast-auth-signer/blob/main/packages/near-fast-auth-signer/src/components/CreateAccount/CreateAccount.tsx