Implement a ValidateInbound method in crosschain keeper

[lumtis]

Describe the Issue
Depends on #2276

As per the specs, ValidateInbound contains logic to validate an inbound and initialize a new CCTX on ZetaChain.

This must be the only entrypoint in order to create a new CCTX

Considered pseudocode:

ValidateInbound(Inbound) {
	// some checks
	fungible.CheckGasExistForChain(Inbound.ChainID)
		
	// init and store the cctx in the store, eventually other actions
	cctx := NewCCTX(inbound)
	saveCCTX(cctx)
	
	// outbound flow
	outboundParams := cctx.GetOutboundParams()
	initiateOutbound(outboundParams)
}

Then use ValidateInbound in all location where we have NewCCTX

• VoteInbound for Observers inbounds
• evm_hook.go for ZEVM inbounds