add option use-base64 for encoded key

[sheng-kenbun]

Hello,

we try to use act to run a workflow locally. The problem is, in act we can not input the ssh-key as secret as multi-lines using the .actrc config file. One solution is, the ssh-key is base64 encoded and can be passed to act using the .actrc file. From the action ssh-agent the string will be base64 decoded before it is passed to the command ssh-agent - ....

This behaviour is controlled by an additional option use-base64.

Please review the changes and make a decision ob this PR should be merged. Thanks!

Best regards,
Li Sheng

[ricbir]

It would be great to have this