go/rpc: add mockery for Client

this help for users of this package
to mock this interface for unit testing

See more documentation here:
https://github.com/vektra/mockery

Example:

```
import (

go/rpc: add mockery for Client

this help for users of this package
to mock this interface for unit testing

See more documentation here:
https://github.com/vektra/mockery

Example:

```
import (
"log"
"errors"
"testing"
"github.com/stretchr/testify/mock"
"github.com/spdk/spdk/go/rpc/client"
"github.com/spdk/spdk/go/rpc/client/mocks"
)

func SpdkGetVersion(c *client.IClient) error {
// a lot of logic before the call
resp1, err1 := c.Call("spdk_get_version", nil)
// a lot of logic after the call
log.Print(resp1)
resp2, err2 := c.Call("bdev_get_bdevs", nil)
// a lot of logic after the call
log.Print(resp2)
return err
}

func Test_Example(t *testing.T) {
mockSpdk := mocks.NewIClient(t)
mockSpdk.EXPECT().Call("spdk_get_version", mock.Anything).Return(errors.New("my error")).Once()
mockSpdk.EXPECT().Call("bdev_get_bdevs", mock.Anything).Return(errors.New("my error")).Once()

err := SpdkGetVersion(mockSpdk)
if err != nil {
t.Error("unexpected error received")
}
}
```

Change-Id: I2747472def0cd73075f52ec678ca45e981bdf0b8
Signed-off-by: Boris Glimcher <Boris.Glimcher@emc.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/20143
Reviewed-by: Jim Harris <jim.harris@samsung.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Artek Koltun <artsiom.koltun@intel.com>
Reviewed-by: Maciej Miś <maciej.mis@intel.com>
Reviewed-by: Ben Walker <ben@nvidia.com>
Community-CI: Mellanox Build Bot

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