Clean up extrinsic params builder

[haerdib]

Changes the following:

• Removed types AssetTipExtrinsicParamsBuilder and PlainTipExtrinsicParamsBuilder. They are simply unnecessary and cause confusion. The api already sets the needed types (Tip, Index and Hash).
• Renamed ExtrinsicParamsBuilder to AdditionalParams. Reasoning: the name "Builder" suggests that it builds "ExtrinsicParams", but it does not. It only ever built the OtherParams. But a builder seems a little over the top anyway. So a simple ...AdditionalParams should do it.
• Renamed Base or Default to Generic because they are compatible with most substrate / polkadot nodes and are generic over Tip and Hash. Base did not fit IMO because that implicates that it is extensible. But it is not. One would need to redefine the Params in case something is changed on the node side.

closes #400

[Niederb]

LGTM, just a few nitpicky suggestions.
Regarding Polkadot vs SubstrateDefault I don't have a strong opinion. I think I would leave it as is for now.

[clangenb]

Nice looks in general good great that you noticed that we can remove the specific builder definitions! I have a few remarks about the terminology:

Renamed Base or Default to Polkadot because they are polkadot-node compatible (and default substrate node). Base did not fit IMO because that implicates that it is extensible. But it is not. One would need to redefine the Params in case something is changed on the node side.

Base might be a bit off, but I think polkadot is wrong because it implies that the params are less generic than they actually are. polkadot-node does not allow AssetTip, while the PolkadotExtrinsicParams do allow this. This is why I chose base, not because it is extensible, but because it is the generic version that can be concretized to polkadot/kusmama/substrate_default or statemine/statemint etc.

So from my point of view we can either name it: ExtrinsicParams (then we need to rename the the ExtrinsicParams trait to ExtrinsicParamsTrait), or we call it GenericExtrinsicParams.

[haerdib]

Renamed to Generic - I think that fits the purpose. Thanks for the input :)

[Niederb]

LGTM

[clangenb]

Thanks for fixing the naming! Looks good to me!