`network prepare`: support mainnets launch

[lumtis]

Depends on #1806

Support mainnets from campaign module in prepare commands (fetch accounts and vesting accounts from the campaign module)

[UPDATE]

Depends on tendermint/spn#789

VestingOptionShares will be disabled on spn

If the chain is a mainnet, we need to call the query implemented in tendermint/spn#789 to get accounts for the mainnet

[lumtis]

After some investigations, this can be simply managed in SPN node without adding complexity

No changes are made to launch a mainnet, the same query will returns the genesis account from the campaign mainnet accounts tendermint/spn#804

[salmad3]

Seems mainnet should not explicitly be supported. Need to determine the usage and clarify objectives regarding SPN.