Merge #1463

1463: Allow for an optional user-defined entry macro when targeting RISC-V r=Dirbaio a=jessebraham

In [esp-hal](https://github.com/esp-rs/esp-hal) we use our own custom runtime crate, [esp-riscv-rt](https://github.com/esp-rs/esp-riscv-rt). This PR adds the ability to optionally specify an entry macro when using `embassy_executor::main`. The following forms are both accepted:

```rust
#[embassy_executor::main] // uses `riscv_rt::entry` by default
async fn main() {}

#[embassy_executor::main(entry = "esp_riscv_rt::entry")]
async fn main() {}
```

I attempted to get this working without needing to quote the entry macro argument, however I was not able to get this working. Based off some reading I did this may not be possible, however I am rather inexperienced with proc macros. Happy to change this if anybody has any insight.

Co-authored-by: Jesse Braham <[email protected]>

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