Support additional **kwargs with default arguments in Transformer API.

[tanujkhattar]

Using the trick originally introduced in #4871 to check for type annotations only in the signature of @cirq.transformer, we are now able to support methods which take additional **kwargs with a default value specified to be valid transformers.

This makes the writing transformers of the following form possible, with type annotations correctly working at both the decorator level and at the call site where the transformer is called using **kwargs.

    >>> @cirq.transformer
    >>> def convert_to_sqrt_iswap(
    >>>     circuit: cirq.AbstractCircuit,
    >>>     context: cirq.TransformerContext,
    >>>     *,
    >>>     atol: float = 1e-8,
    >>>     sqrt_iswap_gate: irq.ISwapPowGate = cirq.SQRT_ISWAP_INV,
    >>>     cleanup_operations: bool = True,
    >>> ) -> cirq.Circuit:
    >>>     pass

This is very useful as it gets rid of the originally discussed constraint of writing a transformer as a class only because it expects additional arguments which can't be supported due to limitations in mypy.

cc @maffoo #4483

[tanujkhattar]

Updated to also change context to a keyword argument and use it's default value declared assigned in the method signature, if none provided.

Fixes #4879

[tanujkhattar]

@maffoo This is ready for re-review. PTAL

[CirqBot]

Automerge cancelled: Failed to update branch (incorrect expected_head_sha).