add vault support

[c9s]

use vault to load the api keys and secrets for the exchange sessions.

[lanphan]

there are 2 ways:

1. based on viper (https://github.com/spf13/viper/pull/497 or Viper extension https://github.com/sagikazarmark/viperx )
2. use lib Go CDK secrets https://gocloud.dev/howto/secrets/ which help us to standardize all secrets interface (Google / AWS / Azure / Viber)

which one you prefer, @c9s ? For quick solution, I suggest to use Viber extension, but needs to check it still works or not (no update in 3 years)