Allow to implement gates up to relative phase

[Cryoris]

What is the expected enhancement?

In several applications we have a compute -> operation -> uncompute flow: we apply a certain series of gates, do some operation, and then apply the daggered series of gates. The gates that are uncomputed can be implemented up to multiplication with a diagonal unitary (a.k.a. up to relative phase). The relative phase version of the gates can have shorter, more efficient implementations than the exact gate. For example the Toffoli gate uses 6 CX gates whereas the relative phase version, sometimes called Margolus gate, requires only 3.

A generic implementation for multi-controlled single qubit gates exists already within Terra, but remains mostly unused. We could add a relative_phase/up_to_diagonal boolean flag to the gate initializers that indicate whether the gate could be implemented up to relative phase. Since we already plan a global_phase keyword (#3472) this doesn't disrupt the gate signature and could significantly simplify complex circuits.
Note that the relative phase CCX and CCCX are already used in computing the multi-controlled Toffoli.

From the implementation point-of-view, the functionality to decompose the relative phase version could become a transpiler pass / synthesizer (@ajavadia) during transpilation.