Add component attributes to builder

[nathanielnrn]

Allows to add attributes to components within the builder. This is needed as part of our AXI-wrapper generator #1733
Something like:

my_comp = prog.component("my_comp")
my_comp.attribute("my_attr", 1)

will produce a component signature that looks like:

component my_comp<"my_attr"=1>(...) -> (...)

I added note of this in the reference doc. Didn't think this warranted changing/adding any tests but can do that if we want.

---

I made sure this works with a little snippet of python uses this, but didn't think it was warranted to go into the repo, included here for completeness:

from calyx.builder import Builder
def add_attr_check(prog):
    my_comp = prog.component("my_component")
    my_comp.attribute("attr1", 7)
    my_comp.attribute("attr2", 1)


def build():
    prog = Builder()
    add_attr_check(prog)
    return prog.program

if __name__ == "__main__":
    build().emit()

emits:

component m_bresp_channel<"attr1"=7, "attr2"=1>() -> () {
  cells {

  }
  wires {

  }
  control {

  }
}

[rachitnigam]

LGTM!