Use CoinType from config as the default coin type

[sorawit]

Closes: n/a. We ran into this issue after #5904 was landed.

Description

In BandChain, we use hardware prefix 494 for BAND token. We want the keyring command to respect the config we set and use 494 as the default value in our case. Otherwise, users will need to put --coin-type all the time.

Proof that this PR works: The following is part of what we get from bandcli keys add --help. You can see that the default is not 494. Before the change it was 118.

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[alessio]

I see your point, though this PR would reintroduce a dependency on the sdk.Config singleton.
I believe CoinType should be a var in types that could be customised via build flags

[alessio]

I take it back - it's out of scope of this PR to solve the sdk.Config conundrum.
ACK from me.