CLI flags and helpers for governance proposals

[SpicyLemon]

Summary

Create some flags and helper functions for commands to use when the command is for submitting a governance proposal.

Problem Definition

With the changes in the gov module, specifically the deprecation of modules adding sub-commands to tx gov, having CLI commands to submit specific governance proposals is left to each module. But all of them will need a common set of options, e.g. deposit, and will always need a MsgSubmitProposal with those fields.

Proposal

1. Create public flag variables for deposit, metadata, title, and summary.
2. Create a helper function that adds them to a command (similar to AddPaginationFlagsToCmd).
3. Create a helper function that reads the flags them and returns a MsgSubmitProposal with appropriate fields populated (similar to ReadPageRequest). This should also populate the proposer field using the --from flag.