Encode submission

This endpoint accepts an EncodeSubmissionRequest, which internally is a
UserTransactionRequestInner (and optionally secondary signers) encoded
as JSON, validates the request format, and then returns that request
encoded in BCS. The client can then use this to create a transaction
signature to be used in a SubmitTransactionRequest, which it then
passes to the /transactions POST endpoint.

To be clear, this endpoint makes it possible to submit transaction
requests to the API from languages that do not have library support for
BCS. If you are using an SDK that has BCS support, such as the official
Rust, TypeScript, or Python SDKs, you do not need to use this endpoint.

To sign a message using the response from this endpoint:

• Decode the hex encoded string in the response to bytes.
• Sign the bytes to create the signature.
• Use that as the signature field in something like Ed25519Signature, which you then use to build a TransactionSignature.