Add support for grayscale bmp image (#14296)

* Add support for grayscale bmp image

This fix tries to address the issue raised in 13942 to support grayscale
bmp image. Previously only channels of 3 or 4 are supported in bmp decoding.
This fix adds the support to have 1 channel grayscale image.

This fix fixes 13942.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add test case for grayscale bmp image decoding

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>