refactor: avoid use of `async_trait` for `RoomIdentityProvider` #13318

Job	Run time
Generate Crypto FFI Apple XCFramework	4m 13s
Run Complement Crypto tests / tests	7m 42s
xtask / xtask-🐧	5s
xtask / xtask-🍏	8s
Test UniFFI bindings generation	1m 49s
matrix-rust-components-kotlin	4m 49s
matrix-rust-components-swift	10m 49s
	29m 35s

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