SecretsManager: list_secrets() now properly splits words when filtering

[bblommers]

Fixes #8421

The documentation states that when filtering by e.g. name or description, the search happens by Prefix match.
When filtering by all (that is, name and description and tags), search approach is:

Breaks the filter value string into words and then searches all attributes for matches.

Until now Moto would split values by spaces (my test -> [my, test]), but AWS uses upper case letters to recognize words (MyTest -> [My, Test]).

Testing this feature showed that filtering by description would also break it into words - it is not a prefix match, as the docs state.

Also introduces proper case insensitivity for the description and all filters.

All new tests are verified against AWS.