[feature] additional recipe metadata

[AlbertDeFusco]

This addresses #4

You can specify recipe metadata keys in following TOML header

[tool.hatch.build.targets.conda.recipe]
...

Recipe metadata is defined in TOML syntax. The metadata is merged using deepmerge with the following strategies

recipe_merger = Merger(
    type_strategies=[
        (dict, ["merge"]),
        (list, ["append"])
    ],
    fallback_strategies=["override"],
    type_conflict_strategies=["override"]
)

Most importantly, this will append list values if the key already exists. For example you can use nested key TOML
syntax:

[tool.hatch.build.targets.conda.recipe]
requirements.run = ["anaconda-anon-usage"]
test.imports = ["my_package"]

will append the package name anaconda-anon-usage to the autogenerated run requirements and inject the test
section into the recipe before it is built.

[AlbertDeFusco]

I'm going to leave this as a draft until #2 is merged