Rename/deprecate num_signatures to num_total_signatures

[apfitzge]

Problem

• The num_signatures function name is confusing since it includes "actual" signatures on the transaction, as well as "pre-compile" signatures that are attached as instructions.

Summary of Changes

• deprecate SantizedMessage::num_signatures
• add new function SanitizedMessage::num_total_signatures
• rename SVMMessage::num_signatures to SVMMessage::num_total_signatures to match

Fixes #

[apfitzge]

Noticed this odd/misleading naming when working on stuff related to #1918

[apfitzge]

Have not deprecated anything in SDK before; is this the correct way to do it?

[bw-solana]

This looks correct to me. Should wait for @buffalojoec to confirm before merging

[apfitzge]

Want to use the new name in #2125; because we need a way to distinguish so that we get the right number to inspect the Signatures on transaction.