Add which tag relative to current index when describe index

[nianiaJR]

Is your feature request related to a problem? Please describe.
During I develope the GUI tool for nebula look-up, I want to know which the tag is relative to the current index I select. It will help me improve the interaction when user uses our look-up function, for example, he selects on index, then our tool will just provide the relative tags for him to select.

Describe the solution you'd like
Nebula has the query like : Describe Tag index index_demo.

// current output

Field	Type
name	string

// idea output

Field	Type
{tag}.name	string

add which tag the field belongs to.

Describe alternatives you've considered
There is another way I can get the relationship between tag and index, but I think it's a little tricky.

By parsing the Create Tag index gql, I can get the tag name too.

[jude-zhu]

u can use SHOW CREATE TAG INDEX statement