feat: multi-hop step accessor safety

Description

Follows-up on https://github.com/shapeshift/web/pull/6765 with getHopByIndex introducing hop access logic consolidation and safety:

Adds a new SupportedTradeQuoteStepIndex so we make accessors safe by only being to access items at 0th and 1st position, since we don't support multi-hops of more than 2 hops currently). This only ensures easier migration to multi-hops consisting of more than 2 hops when we do support that.
Adds a new getHopByIndex that:
- narrows the return type by the input quote being undefined (or not)
- leverages the type-safety above to ensure we only access elements that aren't out of bounds
- ensures we don't try to access an index > 0 for single-hop quotes

Pull Request Type

[ ] :bug: Bug fix (Non-breaking Change: Fixes an issue)
[ ] :hammer_and_wrench: Chore (Non-breaking Change: Doc updates, pkg upgrades, typos, etc..)
[x] :nail_care: New Feature (Breaking/Non-breaking Change)

Issue (if applicable)

relates to the closed https://github.com/shapeshift/web/issues/6764

Risk

High Risk PRs Require 2 approvals

What protocols, transaction types or contract interactions might be affected by this PR?

Very low, this should come as no runtime change, do a small regression of swapper end-to-end as paranoia

Testing

Swapper is still happy for single hops
Swapper is still happy for Li.Fi multi-hops (see PR under this one i.e https://github.com/shapeshift/web/pull/6765)

Engineering

☝🏽

Operations

☝🏽

Screenshots (if applicable)

I'm currently not able to get quotes on this branch, tried both UTXOs and EVMs, with single and cross-account trades.

@0xApotheosis as discussed, way too much "safety" in this PR indeed, voiding very valid accessors at runtime. Fixed in 74ad060060 - and also removed the whole generics dance while at it. The whole if trade quote is defined, hop is defined was actually invalid.

This prompted us to "fix" types in c437ee2da686cdb40220e4ec464f2dd3093eee2 - note fix in quotation marks here as most of this is really explicitly checking for undefineds - or adding non-null assertion for first and last hops - which we were not doing before.

Confirmed single hop quotes are happy again: