Rename the GasPrice Policy to Tips and implement it accordingly.

[MitchTurner]

Before the tx submitter would specify the gas price they wanted via the tx policies but that will now be specified for each block instead. The GasPrice policy will instead be used to specify how much extra the tx submitter wants to spend to incentivise block producers to choose their tx. This is similar to the "priority fee" in EIP 1559. We are choosing to call this a "tip", which is an intuitive description of what it's for.

Part of this is coverage in the metadata tests.

[MitchTurner]

Not necessarily gas price, but maybe fee. Gas price is the rate for gas, tip implies an extra sum.

[MitchTurner]

Do we need another story for including tips in the fee calculation? Because this story replaces the use of GasPrice in fee calc:
#672

[xgreenx]

We need, but I think it will be implemented during #665 (comment)