Better support for full-text searches

[dzhelezov]

Current schema design of full-text search queries is not customizable to support different important uses-cases. Probably the field-level annotation should be replaced with an object-level (say @FullTextQuery) similar to how it is defined in TheGraph schémas. Then it should support

• multiple queries with overlapping field (which may span multiple entities)
• different languages
• Strict/Partial/search-as-you-go queries (only strict node is activated atm)
• support sorting by relevance (ts_rank)

All the options above (and more) are supported by Postgres out-of-the-box so we just need to figure out the right format of the schema definition and translate it into appropriate SQL queries

Related issue: Joystream/hydra#457

[dzhelezov]

Link to TheGraph fts docs: https://thegraph.com/docs/developer/graphql-api#fulltext-search-queries