[feat] Implement OP_CHECKSIGADD

[b-j-roberts]

The last remaining opcode in the engine to be implemented is OP_CHECKSIGADD.

Description
Three values are popped from the stack. The integer n is incremented by one and returned to the stack if the signature is valid for the public key and transaction. The integer n is returned to the stack unchanged if the signature is the empty vector (OP_0). In any other case, the script is invalid. This opcode is only available in tapscript.

NOTE
A large part of the work for OP_CHECKSIGADD had been done as part of this PR, but was closed due to time constraints. This could be a very good start.

We will need to implement the opcode and a set of tests.

References

• PR w/ OP_CHECKSIGADD in shinigami
• OP_CHECKSIGADD impl btcd

[Jeanmichel7]

I'm interested, can I finish it?

[b-j-roberts]

@Jeanmichel7 Definitely, lmk if you have any questions, and good luck 🚀