Default account mechanism for sncast

[cptartur]

Currently, --account must must be always provided in sncast. It can be done through CLI or snfoundry.toml file.

#2221 will allow defining a default account for all sncast projects. This is not very well integrated with our account commands though.

Add an interactive prompt to account import and account create that will prompt the user to make the account they imported default. It should be something like this

> Do you want to set the imported (created) account as default?
Yes
No
Don't ask again

On selecting Yes, user should be informed that the default was stored in global config and how to find/change it.

Additionally, account list should indicate which account is set as a default.

If using sncast command which requires --account, in case the flag is missing, the output should be updated to provide an information about setting the default account.

[codeWhizperer]

@cptartur can I work on this?