refactor: Set defaults in middlewares instead of NewDefaultTxHandler

[amaury1093]

Description

If tx handler options.{ExtensionOptionChecker, TxFeeChecker} is not set, we fallback to a default option provided by the SDK. These default are set in 2 places, which is redundant:

• once in NewDefaultTxHandler
• the second time in the middleware constructor

Proposing here to only set the default once, inside each middleware constructor

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[amaury1093]

same as

cosmos-sdk/x/auth/middleware/fee.go

Lines 31 to 34 in f133317

	func DeductFeeMiddleware(ak AccountKeeper, bk types.BankKeeper, fk FeegrantKeeper, tfc TxFeeChecker) tx.Middleware {
	if tfc == nil {
	tfc = checkTxFeeWithValidatorMinGasPrices
	}

and

cosmos-sdk/x/auth/middleware/ext.go

Lines 35 to 38 in f133317

	func NewExtensionOptionsMiddleware(checker ExtensionOptionChecker) tx.Middleware {
	if checker == nil {
	checker = rejectExtensionOption
	}

[tac0turtle]

lgtm, love the cleanup

[yihuang]

LGTM