event cache: introduce the Paginator API (#3309)

This introduces a new helper object to run arbitrary pagination requests, backwards- or forward-. At the moment they're disconnected from the event cache, although I've put the files there for future convenience, since at some point we'll want to merge the retrieved events with the cache (? maybe).

This little state machine makes it possible to retrieve the initial data, given an initial event id, using the /context endpoint; then allow stateful pagination using a paginator kind of API. Paginating in the timeline indicates whether we've reached the start/end of the timeline.

The test for the state subscription is quite extensive and makes sure the basic functionality works as intended.

Some testing helpers have been (re)introduced in the SDK crate, simplifying the code, and introducing a better `EventFactory` / `EventBuilder` pattern than the existing one in the `matrix-sdk-test` crate. In particular, this can make use of some types in `matrix-sdk`, notably `SyncTimelineEvent` and `TimelineEvent`, and I've found the API to be simpler to use as well.

Part of #3234.

Author: bnjbvr

crates/matrix-sdk/Cargo.toml

@@ -17,7 +17,7 @@ rustdoc-args = ["--cfg", "docsrs"]
 
 [features]
 default = ["e2e-encryption", "automatic-room-key-forwarding", "sqlite", "native-tls"]
-testing = ["matrix-sdk-sqlite?/testing", "matrix-sdk-indexeddb?/testing", "matrix-sdk-base/testing", "wiremock"]
+testing = ["matrix-sdk-sqlite?/testing", "matrix-sdk-indexeddb?/testing", "matrix-sdk-base/testing", "wiremock", "matrix-sdk-test", "assert_matches2"]
 
 e2e-encryption = [
     "matrix-sdk-base/e2e-encryption",
@@ -65,6 +65,7 @@ docsrs = ["e2e-encryption", "sqlite", "indexeddb", "sso-login", "qrcode", "image
 anyhow = { workspace = true, optional = true }
 anymap2 = "0.13.0"
 aquamarine = "0.5.0"
+assert_matches2 = { workspace = true, optional = true }
 as_variant = { workspace = true }
 async-channel = "2.1.0"
 async-stream = { workspace = true }
@@ -91,6 +92,7 @@ matrix-sdk-base = { workspace = true }
 matrix-sdk-common = { workspace = true }
 matrix-sdk-indexeddb = { workspace = true, optional = true }
 matrix-sdk-sqlite = { workspace = true, optional = true }
+matrix-sdk-test = { workspace = true, optional = true }
 mime = "0.3.16"
 mime2ext = "0.1.52"
 rand = { workspace = true , optional = true }

crates/matrix-sdk/src/event_cache/mod.rs

@@ -75,6 +75,7 @@ use self::{
 use crate::{client::ClientInner, room::MessagesOptions, Client, Room};
 
 mod linked_chunk;
+pub mod paginator;
 mod store;
 
 /// An error observed in the [`EventCache`].