Abstract the idea of a `Client` to a trait or traits

[gadomski]

Our backends and our clients are all doing more-or-less the same stuff ... we should abstract those behaviors to a trait or traits.

Initial thoughts (could change, all async):

• SearchClient: search is only required method, item, items, etc all default to using search
• CollectionSearchClient: same thing but for collections
• TransactionClient: add/create/upsert items and collections (do we need to separate into a CollectionTransactionClient?)
• ArrowSearchClient: search_to_arrow iterates over record batches, and provides a default impl of search that does a conversion (like the DuckDB client does now)

We'll want to update several places to use these traits:

• https://github.com/stac-utils/rustac/tree/main/crates/server/src/backend
• https://github.com/stac-utils/rustac/blob/main/crates/api/src/client.rs
• https://github.com/stac-utils/rustac/blob/main/crates/duckdb/src/client.rs

The traits should live in stac-api.