`network status`: Add `mainnet` and `testnet` names to chain contexts

[matevz]

To make sure whether a configured network is Mainnet or Testnet, I suggest we show the network label mainnet or testnet respectively if they are recognized.

Currently:

$ oasis net status --network localhost
=== NETWORK STATUS ===
Network:      localhost
Node's ID:    FtZ8ss0RFPWl8qdzDb8E3oYpayQFu2kIBnb+OzRQgOs=
Core version: 24.3.2

==== Consensus ====
Status:               syncing
Version:              7.0.0
Chain context:        bb3d748def55bdfb797a2ac53ee6ee141e54cd2ab2dc2375f4a0703a178e6e55
Latest height:        23720110 (2025-03-16 22:57:48 +0100 CET)
Latest block hash:    ee4930bbff061249b8616734f51eaa0b1b5c335629b1def09fd5fc747e286465
...

Proposed change:

$ oasis net status --network localhost
=== NETWORK STATUS ===
Network:      localhost
Node's ID:    FtZ8ss0RFPWl8qdzDb8E3oYpayQFu2kIBnb+OzRQgOs=
Core version: 24.3.2

==== Consensus ====
Status:               syncing
Version:              7.0.0
Chain context:        mainnet (bb3d748def55bdfb797a2ac53ee6ee141e54cd2ab2dc2375f4a0703a178e6e55)
Latest height:        23720110 (2025-03-16 22:57:48 +0100 CET)
Latest block hash:    ee4930bbff061249b8616734f51eaa0b1b5c335629b1def09fd5fc747e286465
...