[ioctl] build config get command line into new ioctl

[huof6829]

What would you like to be added

• Step 1: refactor command _configGetCmd into ./ioctl/newcmd/config/config_get.go
• Step 2: create ./ioctl/newcmd/config/config_get_test.go to add unit test

Notes:

1. Use client interface to construct the Cobra command.
2. Output package is deprecated. e.g. output.PrintError could be replaced with errors.Wrap.
3. Global variables (config.ReadConfig config.DefaultConfigFile etc.) have been wrapped into client interface. Please use client interface to access them if needed
4. The logic of new code should be similar to the old one.
5. In each unit test, the results should be checked whether they are as the expected.

Example:

1. https://github.com/iotexproject/iotex-core/blob/master/ioctl/newcmd/account/accountinfo.go
2. https://github.com/iotexproject/iotex-core/blob/master/ioctl/newcmd/account/accountdelete.go
3. https://github.com/iotexproject/iotex-core/blob/master/ioctl/newcmd/account/accountcreateadd.go

Why is this needed

Unit tests couldn't be added for the old ioctl command (./ioctl/cmd)

[pocockn]

I can action this

[huof6829]

I can action this

Welcome! 👍