Support reading in more complex values from query parameters

[joe94]

Context

Currently, Themis can only read in simple claim values from parameters.

For example, today you can tell Themis to "read in the value for this claim from the parameters" like so in config:

claims:
    mac_claim:
        parameter: mac

so when requesting a JWT
curl http://themis/issue?mac=1122334455566

you will get a JWT with the relevant claim:

mac_claim:112233445566

Task

Now, say we want set the value of a claim called "human" to be something like:

{
    "age": 45
    "names: ["Awesome", "Themis"]
}

To do so, the config would stay the same:

claims
    human:
        parameter: human

and on request (subject to change - more experimenting needed)
curl http:themis/issue?human.age=45&human.names=Awesome&human.names=Themis

a claim would be provided like so:

human: 
    age: 45
    names: ["Awesome","Themis"]

Notes:

• From discussions with teammates, it seems like it will be less of an anti-pattern to keep this change localized to parameters and not HTTP headers.

[johnabass]

This would really be better as an HTTP entity body:

curl -X POST -H "Content-Type: application/json" --data-binary '{"human": {"age": 45, "names": ["Awesome", "Themis"]}}' http:themis/issue

[joe94]

So the HTTP entity body would represent the set of claims that needs to be appended to the set of claims Themis builds for a token.

If that's the case, one option is making this the source of highest precedence (that is, body > header > parameter for value overriding). I might have the order for header and parameter flipped here...