[explorer] add testnet toggle in menu bar

Summary:
adding a switch between testnet and mainnet in the navbar
It is off by default, but can be specified in the config.toml for people who want to run it

Test Plan:
add network_selector = true to the config.toml
cargo run in /explorer-exe
check out the switcher

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16724