feat: add try_as_contract fn

[mootz12]

What

Adds a try_as_contract fn that gracefully returns errors like try_invoke_contract.

Why

Fixes #1434

Known limitations

• Marked as draft until the necessary env change is made: feat: add try fn for with test contract frame rs-soroban-env#1628

[leighmcculloch]

Some feedback inline.

[leighmcculloch]

For the return value I think this function should return an InvokeError and the same Result<Result<>, Result<>> structure for consistency with the try_invoke_contract function.

rs-soroban-sdk/soroban-sdk/src/env.rs

Lines 407 to 412 in 9415cee

	pub fn try_invoke_contract<T, E>(
	&self,
	contract_address: &Address,
	func: &crate::Symbol,
	args: Vec<Val>,
	) -> Result<Result<T, T::Error>, Result<E, InvokeError>>

Why: So the dev has the same error handling experience as for invoking. The as_contract fn is basically like calling invoke_contract on a contract function. Also, like the other try_ functions the case where T is the wrong type should be handable as well.

[mootz12]

*_as_contract extracts the result of the closure without converting it to a Val. Thus, I kept the Ok result as T, otherwise we need to convert it to a val and back which felt weird.

Good call on the error, updated it to match the try_invoke method and added tests.