Completion key bindings addition

[frederic-loui]

Most "industry" CLI use mainly 3 shortcuts in order to trigger CLI completion.

1. <Tab> character
2. <Space>character
3. <?> character (mostly triggered in conjunction with <shift>)

JunOS CLI enables the 3 of them above which has the benefit of:

• working nicely with reedline Completer and Menu.
• remove command ambiguity by enforcing complete command syntax at CLI prompt.

-> description handling decided here has to disable <space> completion

IOS-XR/XE enables only [1] and [3] :

• working nicely with reedline Completer and Menu.
• let the user write incomplete cli command sh run instead of show running-configuration

-> description handling decided here has to adjust with <space> usage
The command entered needs to be "normalized" during Validation step.

JunOS style seems to be easier (reedline library check needed) to implement (granted that object description case can be easily handled)

[frederic-loui]

Completion trigger binding added for <?>.
related to issue #2
please note that <space> trigger will be implemented along side in issue #1