feat: support custom mnemonics in in-process testing network (#10922)

## Description

In certain cases, it proves to be beneficial to be able to support user provided mnemonics in the in-process testing network. This essentially allows you to know the validator addresses ahead of time in case you need to do any custom genesis logic (e.g. Gravity Bridge delegation keys).

---

### Author Checklist

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I have...

- [ ] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] added `!` to the type prefix if API or client breaking change
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- [ ] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [ ] added a changelog entry to `CHANGELOG.md`
- [ ] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [ ] updated the relevant documentation or specification
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- [ ] confirmed all CI checks have passed

### Reviewers Checklist

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- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
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- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

testutil/network/network.go

@@ -80,6 +80,7 @@ type Config struct {
 	TimeoutCommit    time.Duration              // the consensus commitment timeout
 	ChainID          string                     // the network chain-id
 	NumValidators    int                        // the total number of validators to create and bond
+	Mnemonics        []string                   // custom user-provided validator operator mnemonics
 	BondDenom        string                     // the staking bond denomination
 	MinGasPrices     string                     // the minimum gas prices each validator will accept
 	AccountTokens    sdk.Int                    // the amount of unique validator tokens (e.g. 1000node0)
@@ -327,6 +328,7 @@ func New(l Logger, baseDir string, cfg Config) (*Network, error) {
 		if err != nil {
 			return nil, err
 		}
+
 		tmCfg.P2P.ListenAddress = p2pAddr
 		tmCfg.P2P.AddrBookStrict = false
 		tmCfg.P2P.AllowDuplicateIP = true
@@ -335,6 +337,7 @@ func New(l Logger, baseDir string, cfg Config) (*Network, error) {
 		if err != nil {
 			return nil, err
 		}
+
 		nodeIDs[i] = nodeID
 		valPubKeys[i] = pubKey
 
@@ -349,7 +352,12 @@ func New(l Logger, baseDir string, cfg Config) (*Network, error) {
 			return nil, err
 		}
 
-		addr, secret, err := server.GenerateSaveCoinKey(kb, nodeDirName, true, algo)
+		var mnemonic string
+		if i < len(cfg.Mnemonics) {
+			mnemonic = cfg.Mnemonics[i]
+		}
+
+		addr, secret, err := server.GenerateSaveCoinKey(kb, nodeDirName, mnemonic, true, algo)
 		if err != nil {
 			return nil, err
 		}