Default RPC endpoint (mynano.ninja) seems to be offline permanently

[kseistrup]

The default RPC endpoint, https://mynano.ninja/api/node has been offline for quite a while, and the hostname doesn't even have an associated IPv4 or IPv6 address anymore.

While this can, of course, be overcome by using the -r/--rpc flag, it may be a hurdle for newcomers and it would be prudent to change the default node to something that works, e.g.:

    https://rainstorm.city/api

See also #15.

🙏

[kseistrup]

PS: To make the fix more resilient, we could introduce and environment variable, e.g. $GONANO_DEFAULT_RPC_ENDPOINT (or something similarly descriptive), such that gonano uses the value of said variable if it has been set, and otherwise falls back to a hardcoded value.

In pseudo code:

if "--rpc" option is used:
    rpc_endpoint="$user_value"
elif "$GONANO_DEFAULT_RPC_ENDPOINT" != "":
    rpc_endpoint="$GONANO_DEFAULT_RPC_ENDPOINT"
else:
    rpc_endpoint="https://rainstorm.city/api"

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edit: typo