Add next_events_from_metadata and rename next_event

[haerdib]

Updated the EventSubscription Wrapper to support decoding events from the metadata:

• added new functionality next_events_from_metadata to derive events from metadata instead of RuntimeEvents generated by the construct_runtime macro of Substrate
• renamed next_event to more appropriate next_events.
• introduced a metadata field for event derivation from metadata (needed by next_events_from_metadata). Therefore, the From derivation from Subscriptionis not possible any more (needs metadata) and was removed.

closes #440

[Niederb]

LGTM

[echevrier]

For me it is not clear what this PR wants to do. Can you clarify?
Why does this PR close #440?
Do we still need RuntimeEvent? When should we use next_events, when next_events_from_metadata?

[haerdib]

Removed because outdated. There's no ac_primitves::Hash Type. No need to point out what Subxt did, because Subxt is constantly evolving as well.

[haerdib]

I updated the PR description, better now?

Regarding the question when to use what: is the comment

substrate-api-client/src/api/rpc_api/events.rs

Lines 124 to 128 in 7536d15

	/// Wait for the next value from the internal subscription.
	/// Upon encounter, it retrieves and decodes the expected `EventDetails`.
	//
	// On the contrary to `next_events` this function only needs up-to-date metadata
	// and is therefore updateable during runtime.

not explanatory enough?

[haerdib]

Do we still need RuntimeEvent

Check out #440 (comment) for more elaborate explanation.
But the short answer would be: no, we don't need it. But I think it would be a waste to not offer it.

[echevrier]

I updated the PR description, better now?
Regarding the question when to use what: is the comment

substrate-api-client/src/api/rpc_api/events.rs

Lines 124 to 128 in 7536d15

	/// Wait for the next value from the internal subscription.
	/// Upon encounter, it retrieves and decodes the expected `EventDetails`.
	//
	// On the contrary to `next_events` this function only needs up-to-date metadata
	// and is therefore updateable during runtime.

not explanatory enough?

It suggests to use every time you can next_events and the next_events_from_metadata when you really need it. Your comment #440 (comment) confirms it. I would like to have this information in the PR description as we do not have examples, which explicitly show the different use cases. This would help the user, without having to click/read the PR and related issues.