Frontend: Implement Context values for Segments

[khvn26]

Blocked on #5674.

DoD

As a user,
When inputting segment condition propertie in the Segment flyout for all operators,
I'm able to choose from the following pre-set values in the Trait input text box:

• Environment Name
• Identifier

As a user,
When inputting segment conditions in the Segment flyout for all operators,
I'm able to enter an arbitrary Trait key in the Trait input text box, in addition to the pre-set Context values.

• The 'is set' and 'is not set' operators should show a warning if the user selects a pre-set context value as they will always evaluate to true / false respectively.

Notes

UX mockup/docs: #5654

[Zaimwa9]

@matthewelwell, to keep you updated, we came up with some UI arrangements with Kim to make it more understandable.
So we can eliminate the tooltip that is quite noisy/intrusive and include it within the dropdown