Add GetVar(inputIndex, varId) + fix Context.getVar

[SethDusek]

Add a new method CONTEXT.getVarFromInput[T](inputIndex, varId) to read other input's context extension values.

Type definition:

def getVarFromInput[T](inputIndex: Short, varId: Byte) -> Option[T] 

Note that if getVarFromInput[T] is called and context variable is some other type U, then it should return None instead of an error, unlike getVar.

Also fix CONTEXT.getVar[T] method, in sigmastate-interpreter the method was implemented but with broken (de)serialization. In sigma-rust the method does not seem to be implemented at all (only global getVar Opcode). Add the getVar[T] method call and serialize T using explicit_type_args (see: #773 for how explicit type args are used)

Reference: ergoplatform/sigmastate-interpreter#1016

[sebastianrof2]

I'm working on it.

[sebastianrof2]

I made the PR for this issue #803.