fix: Make a denom of --min-self-delegation CLI flag required

[youngjoon-lee]

Description

Closes: #9386

As we discussed, I've put a denom to the --min-self-delegation CLI flag.
But, I didn't change the msg types (MsgCreateValidator and so on) which contain MinSelfDelegation as sdk.Int in order not to break the API backward compatibility. If you think that msg types also need to be changed, please let me know :)

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[alexanderbez]

Thanks for the contribution. Can you please format the changes?

[alexanderbez]

This is going to make stake the default flag which I don't think we should do.

[youngjoon-lee]

Yeah. Make sense.
But, I've referred to the defaultAmount which contains sdk.DefaultBondDenom as below:
https://github.com/youngjoon-lee/cosmos-sdk/blob/af995cd5ee5c30dccbb989cd6892e142d5dde818/x/staking/client/cli/tx.go#L24

Because I made a denom required, I think the defaultMinSelfDelegation has to have a denom.
Of course, I added a logic to check whether the denom is the same as the expected one, or not.
What do you think about this strategy?

[youngjoon-lee]

Also, I thought this is a uncompleted improvement, as long as we change MsgCreateValidator to contains MinSelfDelegation as sdk.Coin (not sdk.Int): https://github.com/youngjoon-lee/cosmos-sdk/blob/09f9a5573e4132fbedfaab6d948df1cc93e6e896/x/staking/types/tx.pb.go#L43

While I wrote this PR, I've felt that the original author chose sdk.Int rather than sdk.Coin because he wanted to define MinSelfDelegation as "the minimum value regardless of what its denom is".
If so, I think it's okay to close this PR without merging (It would be better to make consistency between MsgCreateValidator and CLI options).

[alexanderbez]

Yeah honestly, I don't think having a denom in the CLI UX will add that much benefit to be honest. I'm not sure what others think.

[youngjoon-lee]

Yeah. As the Gaia document has been updated: cosmos/gaia#867, I think it's clear that the unit of --min-self-delegation is stake or uatom.

So, I'm closing this PR. Please feel free to open this PR if you think it's necessary. @ryanchristo