Serialize using mapstructure conversion instead of JSON marshalling.

[slewsys]

These changes remove the JSON encoding/decoding steps that are performed during the serialization of ini and dotenv files. This roundtrip loses type information during the transformation which causes values to be incorrectly converted to the JSON marshaller defaults (int becomes float64, bool becomes string, etc, etc). In place of this JSON encoding, the mapstructure library allows for a direct conversion between the Metadata struct and
map[string]interface{} needed to leverage the stores.Flatten and stores.Unflatten functions.

In addition this adds mapstructure tags to the metadata structures to allow backwards compatibility with the JSON encoding.

Resolves #879 & resolves #857

This is a re-submission of @acastle's #1009 with mapstructure updated to v1.5.0 and unused imports removed applied to getsops/sops/main HEAD.

[Ph0tonic]

Hi,
Small tip, if you want this issue to close 2 issues automatically, you need to repeat resolves just before the second issue number, like :
Resolves #879 & resolves #857

[slewsys]

Hi, Small tip, if you want this issue close to issues automatically, you need to repeat resolves just before the second issue number, like : Resolves #879 & resolves #857

Nice. The commit message actually belongs to @acastle, so hopefully we can both benefit! Thank you.

[devstein]

@slewsys Apologies for the slow review time. Thank you for making this PR! I agree using mapstructure is a better solution.

Can you please

1. Rebase to sign off on previous commits (See SO example)
2. Update the branch with the latest

cc @acastle from the original PR.