wallet: enhance multisig file handling and API

[catduck101]

Rebasing and updating #7357 - for reference, that PR added methods to the wallet API for exporting and importing partially signed key images as well as methods for loading and signing multisig transactions from the filename. The development of it basically stalled.

• Improved error handling
• Changed PEM_read_string's return value to a boolean
• Added exportMultisigImagesPEM and exportMultisigImagesToFile for exports and importMultisigImagesFromFile and importMultisigImagesFromPEM for multisig image imports. Removed old unclear functions for importing and exporting images
• dest_addresses renamed to destAddresses in order to be consistent with the camelCase in pending_transaction.cpp and zero copying
• Pass strings by const references in all functions except exportMultisigImagesPEM.
• Fix a PendingTransactionImpl memory leak in loadMultisigTxFromFile

[iamamyth]

It looks to me like the comments on the original PR weren't addressed, and I've effectively just repeated them. Stopping review.

[iamamyth]

One other broad note on this PR: Given the existence and relative priority of the wallet2/wallet_api refactor PRs, this PR will likely remain in the queue for a long time. I think it makes sense to drop PEM support and then just let this stay dormant for a while, as I doubt there'll be many available or interested reviewers.