Keyword search enhancement

[hemphillda]

Provide an ability to specify a list of keywords on an Item to use in item searching. This allows the item to remain generalized but include indexable/searchable information for specific use cases. For example, an item could be used to represent a parcel, county, contract, easement or land use agreement.

This is important for applications using STAC to index remote sensing data (e.g., imagery) across a number of vertical markets. I.e., using the same properties schema but allowing arbitrary searchable keywords.

Usage examples:

3111822110030 (to find an item listing this ID as a keyword)
HEN (to find keywords starting with "Hen")

[matthewhanson]

This has been partially added, by adding mappings for text string for text fields like description and titles in this commit:
3010952

[matthewhanson]

The CQL STAC API extension should cover this.
Implementation covered by #72