Events `2.0`: manually specify signature topic

[ascjones]

How about allowing to manually specify the signature topic? If for whatever reason, the exact definition of types can not be used, we can at least allow to specify the topic manually. We are doing that with custom selectors, why not do it with the event signatures?

Originally posted by @SkymanOne in #1827 (comment)

How it might look:

#[derive(ink::Event)]
#[ink(signature_topic = 0x000...]
pub struct Event {}

The type of the signature topic should be [u8; 32], although we may need to consider making this configurable to Environment::Hash.

[SkymanOne]

What we can do is take some string and hash using blake2x256 or allow the user to specify the hash manually.

[SkymanOne]

How it might look:

I think the best approach is to keep it consistent with custom selectors:

#[ink(event, signature_topic = 0xDEADBEEF)]
pub struct Event {}

or

#[ink::event]
#[ink::signature_topic = 0xDEADBEEF]
pub struct Event {}

[ascjones]

For ink::event it should be an option e.g.

#[ink::event(signature_topic = 0xDEADBEEF)]
pub struct Event {}