Improve plugin declarations and their usage

[nazarhussain]

Description

We need couple of refactoring around plugins and their declarations identified from the earlier PR.

1. Make plugin name a property rather the getter.
2. Rename function overridePluginOptions to updatePluginConfig to be consistent across naming convention.
3. Rename all files in plugins defaults/config.ts to cofnigSchema.ts or schema/config.ts
4. Don’t need the code objects.mergeDeep in every plugin load its already done in the base plugin.
5. Need to remove events and actions from abstract members and create properties with default values.
6. If possible create an issue to remove dependency of declaring nodeModulePath in every plugin.
7. Two functions _loadInMemoryPlugin and _loadChildProcessPlugin have extra parameter name. which can be removed now. Earlier it was the feature to register a plugin with different alias, now we restrict everywhere to just use plugin instance name.
8. Remove the use of assert around new code case

Motivation

Make plugin declaration and usage seamless and with minimum effort for Lisk SDK user.

Additional Information

For more details the discussion on this RR can be checked.

#6664 (review)