refactor: avoid use of `async_trait` for `RoomIdentityProvider` #13313

Job	Run time
Generate Crypto FFI Apple XCFramework	4m 32s
Run Complement Crypto tests / tests	8m 14s
xtask / xtask-🐧	4s
xtask / xtask-🍏	8s
matrix-rust-components-swift	10m 20s
Test UniFFI bindings generation	1m 53s
matrix-rust-components-kotlin	4m 34s
	29m 45s

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