Integrations: Add UI component for ENV-like key-values

With !79665 (merged) we added an integration that has as one of its properties a key-value mapping, stored as a string.

This has UX downsides, such as needing to do parsing and validation, and it makes it harder for users to see that what they have entered is obviously correct.

Idea:

introduce a UI component for integrations that lets users specify key/value pairs, using pairs of text boxes. This component should support cut-and-paste of string values, to support simplicity of operation.