Ledger server is configurable by ENV variable "LEDGER_SERVER_URL"

[willy-b]

• Submitted a ticket for my issue if one did not already exist

This supports #4286 (since this makes it easy to point Brave to a Ledger server using Bitcoin testnet)

• Used Github auto-closing keywords in the commit message.

[willy-b]

Test plan:

1. Clear local Ledger data
2. Check in browser if my unreliable testnet Ledger server is available: http://ledger-testnet.entropi.info/documentation
3. If it is, run Brave with `LEDGER_SERVER_URL=http://ledger-testnet.entropi.info'
4. Open Preferences->Payments
5. Confirm Wallet can be created
6. Grab wallet address (from Add Funds)
7. Visit a testnet faucet (https://testnet.manu.backend.hamburg/faucet)
8. Confirm testnet bitcoin can be transferred to address

for #4286

[mrose17]

lgtm.

[willy-b]

thanks for the review+merge @mrose17!

[luixxiul]

I am testing it out and it looks working well. I had to delete the local ledger-* files at first (as mentioned in the post above).

@willy-b it would be nice if it becomes possible to avoid the 30 min waiting time and funds are added instantly.

[luixxiul]

Though the wallet became ready, some kind of error is happening.

[luixxiul]

Nevermind, I just restarted brave without the env. it works now 👍