Add an oc.decode resolver to parse and evaluate strings

[odelalleau]

Is your feature request related to a problem? Please describe.

With env being deprecated in favor of oc.env (#573), which always returns environment variables as strings, we need an easy way to convert strings to common types like bool, int, float, etc.

Describe the solution you'd like

Introduce a new resolver oc.decode that converts a string automatically, using the common OmegaConf grammar.
For instance:

cfg = OmegaConf.create(
    {
        "database": {
            "port": '${oc.decode:${oc.env:DB_PORT}}',
            "nodes": '${oc.decode:${oc.env:DB_NODES}}',
        }
    }
)

os.environ["DB_PORT"] = '3308'  # integer
os.environ["DB_NODES"] = '[host1, host2, host3]'  # list

print(repr(cfg.database.port))
print(repr(cfg.database.nodes))

should print:

3308
['host1', 'host2', 'host3']

In addition, ${oc.decode:null} is valid and returns None, in order to support for instance ${oc.decode:${oc.env:DB_PORT,null}}

Differences with the old env resolver

Note that ${oc.decode:${oc.env:VAR}} is not strictly equivalent to the old ${env:VAR} from OmegaConf 2.0. This is because oc.decode relies on the same parsing logic as used when parsing resolver arguments in interpolations, while env was using more basic custom parsing functions. Some key differences are:

• oc.decode will raise an exception if the content of the environment variable is invalid according to the grammar (ex: an unquoted string containing invalid characters). One may use quotes (ex: VAR="'~!&*'") to avoid such exceptions (or just not use oc.decode to keep the original content of the variable)
• oc.decode handles more data types like dictionaries, lists, null and even interpolations, which would have been kept as strings when using env