feat: make the bnb sdk an optionalDependency

[mrnerdhair]

• We don't use BNB support in shapeshift/web at the moment, and having to ship a heavy SDK we don't use is silly
• The old bnb-javascript-sdk-nobroadcast is very stale and isn't maintained under the org (I made @shapeshiftoss/bnb-javascript-sdk to fix that, it's got CI and everything)
• The BNB SDK version includes various transitive dependencies which produce npm audit warnings. None of these are applicable to our specific use of it, but providing a way for other devs not to have to depend on this package -- and therefore get bugged by their users -- would be a nice-to-have, especially if they only want e.g. Ethereum support in the first place.

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Name	Status	Preview	Updated
hdwallet	✅ Ready (Inspect)	Visit Preview	Jun 9, 2022 at 7:02AM (UTC)

[0xean]

closing as stale.