We should support string indexes for all marker string types

[mstange]

To save space in the profile, we can deduplicate strings in marker fields by using the unique-string field type. However, how do I use deduplicated URLs? Or deduplicated sanitized strings? I have to pick either url or unique-string, there's no unique-url or unique-sanitized-string:

https://github.com/firefox-devtools/profiler/blob/af62509321b9610b7acacb7e4094eccd99945a35/src/types/markers.js#L20-L37

I think it would be better to remove the unique-string type and turn all string fields into string indexes.

┆Issue is synchronized with this Jira Task