Remove the use of Custom types

[Nakul]

Is your feature request related to a problem? Please describe.
Currently the runtime/utils contains custom implementation for AccountId and MultiSignature. This is done because scale_encode::EncodeAsType and scale_decode::DecodeAsType are not implemented for AccountId which is required for subxt version 0.29.0. But it adds an upstream dependency on subxt/utils.

Describe the solution you'd like
An ideal solution would be to use sp_core crate and wrap the custom types in Static. As Static by default implements scale_encode::EncodeAsType and scale_decode::DecodeAsType.

Additional context

• Issue #1040 on subxt shades more light on why the specific traits are needed.