keys: tell the user where a key is saved

[leighmcculloch]

What

Output to users the path that the key is written to when added or generated.

Add:

$ stellar keys add me --seed-phrase
Type a 12 word seed phrase: 
*****
✅ Key saved with alias "me" in "/Users/leighmcculloch/Code/stellar-cli/.soroban/identity/me6.toml"

Generate:

$ stellar keys generate me --fund
✅ Key saved with alias "me" in "/Users/leighmcculloch/Code/stellar-cli/.soroban/identity/me8.toml"
✅ Account "me" funded on "Test SDF Network ; September 2015"

Fund:

$ stellar keys fund me
✅ Account "me" funded on "Test SDF Network ; September 2015"

Why

Close #1810

Known limitations

[TODO or N/A]