Update WithExtrinsicParams to use ExtrinsicSigner<Self>

[haerdib]

• Updates WithExtrinsicParams to use the ExtrinsicSigner<Self> instead of the T::ExtrinsicSigner, which is defined as ExtrinsicSigner<Self> but with the other runtime.
• Simplifies examples to use into() instead of more verbose ExtrinsicSigner::new
• Use ExtrinsicSigner with DefaultRuntimeConfig on chain_tests.rs. Reasoning: Does not need a real signer, but it still ensures that DefaultRuntimeConfig is tested atleast as a Getter.

Fixes #628

[masapr]

LGTM 👍