add device-os endpoint support

[bwJuniper]

POST /api/device-os/images-external

[bwJuniper]

{
platform string
enum: CUMULUS | JUNOS | NXOS | SONIC | EOS
Device OS platform. Supported platforms are: ['EOS', 'NXOS', 'CUMULUS', 'JUNOS', 'SONIC']

description string
minLength: 1
Device OS image description

image_url string
minLength: 1
External URL to the device OS image file

checksum string
default: null (Optional)
String representation of the SHA512/SHA256/MD5 checksum.

checksum_type string
enum: MD5 | SHA256 | SHA512
default: null (Optional)
}