feat: fungible proxy

[yomarion]

Smart contract to handle fungible token transfers with no conversion.

[MantisClone]

Hey @yomarion.

1. Please add the new fungible_proxy tests to the GitHub Action.
2. The fungible_proxy tests are failing with the following error:

   mantisclone@mantisclone-Inspiron-7620:~/projects/near-contracts/fungible_proxy$ ./test.sh 
          Compiling fungible_proxy v0.0.1 (/home/mantisclone/projects/near-contracts/fungible_proxy)
   error[E0560]: struct `PaymentArgs` has no field named `amount`
      --> fungible_proxy/src/lib.rs:240:13
       |
   240 |             amount: 1000000.into(),
       |             ^^^^^^ `PaymentArgs` does not have this field
       |
       = note: available fields are: `fee_address`, `fee_amount`, `payment_reference`, `to`
   
   For more information about this error, try `rustc --explain E0560`.
   error: could not compile `fungible_proxy` due to previous error
   

3. The top-level test.sh fails with the following error:

   error: linking with `cc` failed: exit status: 1
   ...snip for brevity...
     = note: /usr/bin/ld: cannot find -lstdc++: No such file or directory
             collect2: error: ld returned 1 exit status
             
   
   error: could not compile `request_conversion_proxy` due to previous error
   

[yomarion]

Hey @yomarion.

1. Please add the new fungible_proxy tests to the GitHub Action.
2. The fungible_proxy tests are failing with the following error:

   mantisclone@mantisclone-Inspiron-7620:~/projects/near-contracts/fungible_proxy$ ./test.sh 
          Compiling fungible_proxy v0.0.1 (/home/mantisclone/projects/near-contracts/fungible_proxy)
   error[E0560]: struct `PaymentArgs` has no field named `amount`
      --> fungible_proxy/src/lib.rs:240:13
       |
   240 |             amount: 1000000.into(),
       |             ^^^^^^ `PaymentArgs` does not have this field
       |
       = note: available fields are: `fee_address`, `fee_amount`, `payment_reference`, `to`
   
   For more information about this error, try `rustc --explain E0560`.
   error: could not compile `fungible_proxy` due to previous error
   

3. The top-level test.sh fails with the following error:

   error: linking with `cc` failed: exit status: 1
   ...snip for brevity...
     = note: /usr/bin/ld: cannot find -lstdc++: No such file or directory
             collect2: error: ld returned 1 exit status
             
   
   error: could not compile `request_conversion_proxy` due to previous error
   

Thanks @MantisClone , I've fixed points 1. and 2.

About point n.3 maybe a missing dependency on your local environment? Don't you have the same problem on master? The test.sh script does not do much and both build and tests work in GH workflows

[MantisClone]

Great looking PR!