Support map values and nested values for attributes

[tigrannajaryan]

After #368 gets merged we will have support for array values.

If we add support for maps and nesting it will allow to represent arbitrary nested data structures in attribute values if needed.

This will apply to span and resource attributes.

[jmacd]

Can we explicitly state that this applies to resources too? I believe that span attributes and resources (which are only specified in the proto, currently) are specified with the same structure.

[Oberon00]

Are there any use cases for arbitrary nesting? I think (multi)maps would be useful to store, e.g., HTTP headers, but what would be the rationale for arbitrary nesting?

[shengxil]

Arbitrary nesting map can represent the classified values e.g. {"http" : {"url":...,"method":...}} or {"sql" : {"query":...,"engine":...}}. It can also host vendor specific data like {"aws": {"account_id":...}} in the situations when Resource isn't the right place, e.g. client side metrics

[jmacd]

In #579, Tigran's example seems to contain a use-case. The resource of "application B" is a set of key-value attributes.