Add KMS functionality

[torao]

Description

This PR enables the use of a secure KMS (Key Management System) from Ostracon.

Currently, the key pairs used by Ostracon for agreements are stored locally. But Ostracon nodes are at risk of having their private keys stolen when a malicious attacker breaks into the system since they must accept inbound connections from the Internet.

For more mission-critical applications, such as finance, the node storing the private key must be independent of the Ostracon agreement node, placed behind a firewall, and able to connect to solutions using an HSM (Hardware Security Module) or our SGX solution.

Tendermint, a fork of Ostracon, has the ability to connect to KMS using tmkms communication protocol. Note, however, that the init and unsafe-reset-priv-validator subcommands of the Tendermint CLI are not designed to configure KMS, and this PR still requires copying the public key stored in the KMS to a local key file. Thus, you should be set up as follows if you use your KMS:

1. Create an initial configuration with the ostracon init command.
2. In ~/.ostracon/config/priv_validator_key.json, replace pubkey/value field with the Base64 representation of the public key stored in KMS.
3. In ~/.ostracon/config/genesis.json, replace the validators/address and validators/pubkey/value fields with the values from the KSM public key in the same way if the node is the initial validator.

This PR contains the following changes:

• Connection test with KSM at the library level. This is a program that starts with main() instead of a unit test.
• ostracon run: Removed the behavior of referencing local key pair when priv_validator_laddr is set.
• ostracon show-validator: Changed to get and display KSM public key when priv_validator_laddr is set.

[codecov]

Codecov Report

Merging #417 (dd07f9b) into main (f19316f) will increase coverage by 0.19%.
The diff coverage is 93.47%.

@@            Coverage Diff             @@
##             main     #417      +/-   ##
==========================================
+ Coverage   65.28%   65.48%   +0.19%     
==========================================
  Files         277      278       +1     
  Lines       37842    37926      +84     
==========================================
+ Hits        24707    24835     +128     
+ Misses      11321    11285      -36     
+ Partials     1814     1806       -8     

Impacted Files	Coverage Δ
cmd/ostracon/commands/show_validator.go	72.72% <82.35%> (+72.72%)	⬆️
config/toml.go	74.19% <100.00%> (+0.86%)	⬆️
node/node.go	60.12% <100.00%> (+0.87%)	⬆️
privval/test_util.go	100.00% <100.00%> (ø)
statesync/snapshots.go	93.71% <0.00%> (-1.26%)	⬇️
mempool/reactor.go	78.57% <0.00%> (-1.10%)	⬇️
p2p/switch.go	65.90% <0.00%> (-0.62%)	⬇️
light/client.go	61.61% <0.00%> (-0.45%)	⬇️
consensus/reactor.go	75.02% <0.00%> (+0.08%)	⬆️
consensus/state.go	73.96% <0.00%> (+0.29%)	⬆️
... and 7 more