Create AddGovPropFlagsToCmd and ReadGovPropFlags.

[SpicyLemon]

Description

This PR:

1. Makes the FlagMetadata variable public.
2. Creates an AddGovPropFlagsToCmd func that adds FlagDeposit, and FlagMetadata to a cobra command.
3. Creates a ReadGovPropFlags func that creates a MsgSubmitProposal pre-populated with provided flag values and proposer (from address).

It's similar to cosmos#14718 except doesn't have the FlagTitle, and FlagSummary yet since that's not yet in our fork.

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[derekadams]

LGTM