Changes in `validateAddress` function, tests & docs

- Replaced `filter` with a `for..of` loop when retrieving the `network`.
- Replaced `new TypeRegistry` with `this.api.registry` when retrieving the `accountID`.
- Renamed `networkName` to `network` so it is aligned with the key naming in the SS58 registry.
- Updated the JSDocs of the function `validateAddress`.
- Added the default mock api into the service in the tests.
- Updated the docs.