Add metadata to sweeps

[dstrain115]

• Adds optional metadata attribute to Linspace and Points.
• This will be used to adjust device parameters via sweep.

See https://tinyurl.com/cirq-sweep-metadata-public for RFC.

[maffoo]

Minor comments, then LGTM

[maffoo]

Should probably add metadata on ListSweep as well, since that also can be serialized and sent to the API (https://github.com/quantumlib/Cirq/blob/master/cirq-google/cirq_google/api/v2/sweeps.py#L60).

[maffoo]

Note that in the ListSweep case, the sweep is converted during serialization into a zip over individual Point sweeps when serializing, so we probably want the ListSweep metadata to be something like Mapping[cirq.TParamKey, Any] so that the metadata can be split up appropriately. Similarly in the "flatten expressions" transformer we will want to be able to collect metadata about the individual param keys into a map and attach it to the ListSweeps that are produced.

[dstrain115]

Ok, thanks for the corrections. I will save ListSweep for a follow-up PR. I want to give it a bit more though.