Allow searching for nested keys in consul UI

#### Feature Description

When keys are nested inside of folders, it becomes hard to find or search for them on the key/value page in consul UI. This feature would display nested keys if they match the search term. It could be done as an additional search option (eg, a checkbox next to the "Type" dropdown). I think that matching any part of the path should count.

Alternatively, there could be an option to "flatten folder structures" or something, so that if there were two keys `folder/key1` and `folder/key2`, they would both be displayed on the page instead of just displaying `folder`. If folders are flattened, then the logical way to search would include nested keys anyway, since you would be able to see the full path.

#### Use Case(s)

Easier searching for keys while maintaining the advantages of a folder structure

Start at the Consul UI key/value page and inspect the existing search behavior and the “Type” dropdown. Decide whether the implementation should search nested key paths or flatten folder structures, then define the matching and display behavior for keys such as `folder/key1` and `folder/key2`. Done means nested keys can be found or displayed according to the selected behavior.

Written by the indexing model from the issue text.