Fixes DISCO-2470: Implement the foundation of a Firefox Suggest component #5723

This commit adds a new component to fetch search suggestions from Remote Settings and store them in an SQLite database.

Before, creating a provider would block the calling thread until the database was set up. Now, migrations will run the first time the database is accessed, which we already expect to happen on a background thread.

This commit adds an empty UniFFI interface for the new Suggest component.

In other uses, "provider" refers to the source of a suggestion, like adM, Wikipedia, AMO, etc. Since `SuggestionProvider` aggregates suggestions from all these sources, calling it a "provider" could be confusing, especially if we support filtering suggestions from specific sources in the future.

The Gecko JS UniFFI bindings don't support synchronous function calls yet (https://bugzilla.mozilla.org/show_bug.cgi?id=1842433), so the builder API will be a little awkward to use on Desktop. This commit replaces the builder with a dictionary, with default fields to make it more ergonomic.

This commit adds documentation and tests for `full_keyword`, and reimplements it using the Rust iterator methods.

This commit exposes `SuggestDb.clear()` as `SuggestStore.clear()`, and ensures that it removes all icons in addition to suggestions and metadata.

This commit changes the visibility of crate-internal types and methods from `pub` to `pub(crate)`, and clarifies the difference between the two error types: * `SuggestApiError` is the public error type, for all errors that should be handled by the application. * `Error` is the internal error type, used in the implementation of the component, and is not part of the public interface.

This commit makes it possible to combine multiple database accesses into a single transaction. Previously, each `SuggestDb` method would lock the database connection, and optionally run its own transaction before releasing the lock. This meant that a single logical operation, like ingesting all suggestions from an attachment, would involve multiple transactions. This commit moves the database accesses into a new `SuggestDao` type, which takes a reference to an existing locked connection. The new `SuggestDb::{read, write}` methods handle locking and transaction management, and take a closure that receives a `SuggestDao` for reading and writing. Co-authored-by: Tif Tran <ttran@mozilla.com>

`SuggestDb` already protects the connection with a Rust mutex, so SQLite's per-connection mutex is unnecessary.

This commit also generalizes `IngestLimits.records` to `SuggestIngestionConstraints.max_suggestions`, since the partitioning scheme and number of suggestions per Remote Settings attachment is an implementation detail.

"Fetching" is a bit of an overloaded term that can refer to querying ingested suggestions, or downloading records from Remote Settings. To reduce confusion, let's replace those uses with more precise terminology: * "Downloading" for when we request suggestions from Remote Settings. * "Ingesting" for storing the downloaded suggestions in the database. * "Querying" for when the application requests ingested suggestions.

This matches how we pass dictionaries across the FFI boundary in our other components.

A bit of future-proofing, in case we support multiple Remote Settings collections in the future.

This commit renames: * `RemoteSuggestion` to `DownloadedSuggestion` and `SuggestDataAttachmentContents` to `DownloadedSuggestDataAttachment`, for consistency with the "downloading" terminology. * `RemoteRecordId` to `SuggestRecordId`, for consistency with the other Remote Settings types.

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Fixes DISCO-2470: Implement the foundation of a Firefox Suggest component #5723

Fixes DISCO-2470: Implement the foundation of a Firefox Suggest component #5723

Commits on Jul 20, 2023