feat: adding mint address validation at create (#17)

Fixes HAL-002

> ## Description
> After the user invokes the process_init entry point, they need to
invoke the process_create entry point to store the necessary information
to participate in the token locking process.
> 
> A piece of information that the user has to provide is the source
token account, where the tokens will be subtracted to participate.
> 
> The mentioned token account is not validated to be a token account
corresponding to the L3 token, which is expected to be the token used.
> 
> As a result, any user can create a vesting_account that does with any
token different than L3 token.
> 
> The L3 team mentioned that this situation does not represent a risk,
since the information on chain will be used on an off-chain process.
> 
> However, it is considered a good practice to restrict the
functionalities of a program used in production to narrow down the
attack surface in order to prevent unexpected behaviors.

program/src/processor.rs

@@ -23,6 +23,9 @@ use crate::{
     state::{pack_schedule_into_slice, unpack_schedule, VestingSchedule, VestingScheduleHeader},
 };
 
+pub const VALID_TOKEN_MINT: Pubkey =
+    solana_program::pubkey!("AxfBPA1yi6my7VAjqB9fqr1AgYczuuJy8tePnNUDDPpW");
+
 pub struct Processor {}
 
 impl Processor {
@@ -90,6 +93,12 @@ impl Processor {
         mint_address: &Pubkey,
         schedule: Schedule,
     ) -> ProgramResult {
+        // Validate the mint address matches the expected token address
+        if *mint_address != VALID_TOKEN_MINT {
+            msg!("Unsuported token mint address");
+            return Err(ProgramError::InvalidArgument);
+        }
+
         let accounts_iter = &mut accounts.iter();
 
         let spl_token_account = next_account_info(accounts_iter)?;