Implement Hash for Schema

[crepererum]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Schema currently implements Eq but not Hash. The latter would be useful for the following reasons:

1. Sometimes it would be helpful if you could stick a Schema into a HashSet/HashMap, e.g. for interning (this is a quite costly way to intern a schema and usually you should use a cheap proxy key, but sometimes this is also the only easy way to intern a schema)
2. Implement Hash for a struct that contains a Schema for whatever reason.

Describe the solution you'd like
Implement Hash for Schema.

Describe alternatives you've considered
Not implementing it.

Additional context

• IOx PR where this could be used: https://github.com/influxdata/influxdb_iox/pull/5215