authz/ics20: review comments for initial PR

[crodriguezvega]

Initial PR.

• Rename allowed_addresses to allow_list and use in the protodoc the same comment as SDK in authz.proto for send. @damiannolan
• Make IsAllowedAddress private. @chatton Make isAllowedAddress private #2985
• Rename PortChannelAmount to Allocation. @damiannolan
• Rename transfer_authz.go to transfer_authorization.go and transfer_authz_test.go to transfer_authorization_test.go. @chatton Renaming transfer authorization files #2990
• Rename ErrDuplicateEntry to ErrInvalidAuthorization and change the error text to invalid transfer authorization (see this comment). @chatton Change ErrDuplicateEntry to ErrInvalidAuthorization & change error formatting #2988
• Replace usage of this constant with SDK's. constant is not exported, would require changes to the SDK to do this
• Format errors returned as sdkerrors.Wrap(<type>, <message>) instead of sdkerrors.<type>.Wrapf(<message>). @chatton
• Move authz.proto to proto/ibc/applications/transfer/v1. @damiannolan
• Convert NewTransferAuthorization into a variadic function: @damiannolan

func NewTransferAuthorization(allocations ...Allocation) *TransferAuthorization {
	return &TransferAuthorization{
		Allocations: allocations,
	}
}

Separate PRs against the feature branch can be opened for each of these items.

[damiannolan]

Will tag myself beside tasks as I pick them up