Support for searching by link and asset via synthetic queryables

[philvarner]

In the way I use it here, a synthetic queryable is one that doesn't exist as a literal field in the Item. Conceptually, all queryables are synthetic, as there is a mapping defined between the queryable term (e.g., eo:cloud_cover) and the search filter this translates to (e.g., item.properties.eo:cloud_cover <= 32.1), but synthetic is applied here to mean that that there's a more complex relationship expressed in the mapping.

Examples of what would be supported are:

1. The Item has an Asset with the key red.
2. The Item has an Asset with an eo:band array containing red and has a role array containing data.
3. The Item has a Link with link relation derived_from and an href of https://planetarycomputer.microsoft.com/api/stac/v1/collections/sentinel-2-l2a/items/S2A_MSIL2A_20230320T052641_R105_T48XWH_20230320T082142