[API][v1] Unify MIME types

[banool]

Currently we use a mix of mime types for BCS:

• application/x-bcs
• application/x.aptos.signed_transaction+bcs
• application/x.aptos.output+bcs

I think we should just cut over to using only the first one, I don't think it's necessary to signal more information with the content type.

[gregnazario]

There's one for input and one for output now, and we'll stick with it for now

[ghost]

aptos-core/api/src/transactions.rs

Lines 110 to 112 in a34544e

	// TODO: Since I don't want to impl all the Poem derives on SignedTransaction,
	// find a way to at least indicate in the spec that it expects a SignedTransaction.
	// TODO: https://github.com/aptos-labs/aptos-core/issues/2275

👀 the todo comment still exists on this moment now.