Segment list should not be created for feature flag which doesn't exist

[Yagnik56]

Issue:
In the add list, we can only select a segment if it exists with the same context as the feature flag, but while importing, we haven't applied any validation to make sure the segment list that is getting created already exists. This might apply to groups as well.

(For this case warning should be shown on import while creating but was not showing)

[RidhamShah]

A validation was also added to check multiple imports of duplicate files as the second file will try to create on key already used and throw an error.

[Yagnik56]

segment list should be ignored if it doesn't exist but here that's not happening and it is getting created.
Also, the segment list validation is just checking the id and attaching a list to a segment with a different context which shouldn't be allowed.

video1844997364.mp4

[ppratikcr7]

I will QA this

[ppratikcr7]

QA passed