Add global config for default network and identity.

[fnando]

What

This pr introduces a global config file called ~/.stellar/config.toml. Right now, it contains a new section called defaults, which can hold the values set by the new commands stellar keys use NAME and stellar network use NAME. An upcoming pr will use this values by default, when available.

$ stellar network use testnet
ℹ️ The default network is set to `testnet`

$ stellar keys use alice
ℹ️ The default source account is set to `alice`

$ stellar env
STELLAR_NETWORK=testnet # default
STELLAR_ACCOUNT=alice   # default

$ cat ~/.config/stellar/config.toml
[defaults]
network = "testnet"
identity = "alice"

Why

This is the first step to implement #1396.

Known limitations

N/A