Add the ability to pass Parquet tags on map key and values

[Pryz]

Fixes #256

[achille-roussel]

We discussed offline the option to use a different struct tag name for the key, e.g. key, mapkey or parquet-key, which has advantages over the key= prefix:

• the grouping of attributes for the map key help with readability
• there is no need for repetition like there is with the key= prefix

Did you get a chance to give this more thoughts?

[Pryz]

Did some experimentation and went with the parquet-key tag.

Example:

type Spans struct {
    Attributes map[string]json.RawMessage `parquet:"attributes,optional,json,zstd" parquet-key:"zstd"`
}

[Pryz]

Thanks for the review !