feat: p-chain dynamic fees

[erictaylor]

This is a first draft of changes needed for upcoming p-chain dynamic fees.

This PR introduces new utils and a new builder that is currently exposed alongside the existing builder. All new builder functions are currently exported under a etna specific directory.

The new builder does not include any transactions types that are not supported post Etna upgrade.

Tests are added that align for the most part to the tests in avalanchego. However, the builder tests are a bit of a hybrid between existing builder tests and new changes needed for dynamic fee testing.

Things this PR does not do (will come later)

• Pulling gas price and weights from API to set in context.
• Calculating "compute" complexity of transactions.

Things not planned (currently)

• This does not touch the X-chain. We are following in avalanchego's steps here, and the X-chain hasn't been worked on. Though the idea is that the changes will look very similar and there could be a lot of code reuse.

Extra changes that do not effect current implementation.

• Changed a bunch of types (for arrays) to be readonly. Mostly to keep my sanity that certain values were immutable. I wanted that extra piece of mind. All of the places readonly is added it doesn't effect existing implementations and was already being treated as such.
• Fixing some spelling mistakes as I was going along reading existing code.
• Adds a cspell config but does not enforce it. Also added the CSpell extension recommendation to VSCode. Helps with spelling mistakes. ;)

[rictorlome]

Thank you, this is really coming along! I left some more notes

[bferenc]

Awesome work! Haven't checked everything yet, but wanted to stop and share a few questions regarding the useUnlockedUTXOs. Please take a look at my comments below when you have a moment 👇

[bferenc]

Awesome work! 👏

[rictorlome]

Great job!