chainHead: Sync events with spec (#14654)

* chainHead/events: Remove network config

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* chainHead/events: Add events for operations

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* chainHead/tests: Test chainHead events serialize/deserialize

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* chainHead/events: Remove generics from the storage event

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* chainHead/events: Rename kebab-case to camelCase

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* chainHead/events: Add methodResponse object

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

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Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>