As a user, I want to be able to sign arbitrary XDRs

[fsodano]

With the introduction of extra signers https://developers.stellar.org/docs/glossary/multisig#extra-signers we should be able to sign arbitrary XDRs, and because they are arbitrary and not required to build the XDR in the first place, we should build a flow where the user can connect the wallet first and then sign the XDR in one seamless step.