SanitizedTransaction is too burdensome to construct

[LucasSte]

When submitting a transaction to SVM, one must create a SanitizedTransaction, but all its fields are private. Although it has a try_new function, it depends on SanitizedVersionedTransaction, whose try_new construction, in turn, depends on VersionedTransaction.

Such a process is so onerous that we use SanitizedTransaction::from_transaction_for_tests in our tests.

All these levels of indirection ensure the safety for the Solana use-case, but they create a burden for external users who need to navigate all these intricacies just to create a SanitizedTransaction.