[FEATURE] Add support for `defcal`

[TheGupta2012]

Feature Description

Add support for pulse level description of quantum gates using the defcal gate definitions.

Reference : https://openqasm.com/language/pulses.html#pulse-level-descriptions-of-gates-and-measurement

Implementation (Optional)

Given that the grammar used to define defcal blocks is openpulse, we will need to figure out how to validate the semantics of these blocks. This is because the openqasm AST object treats the defcal body as a string and not as a parsed Python object.