feat: allow multiple header values in upstream headers configuration

[Prototik]

First of all: thanks for this awesome plugin!

I'm integrating oidc auth in some app which uses kubernetes api and was required to pass multiple values for a header due to how user impersonation works on kubernetes. Unfortunately, this project lacks this ability... until now I guess.

To pass a list of values through all that yaml stuff I couldn't think anything better to pass a template which produces a json array, which values I set to header values. If anyone have a better idea - welcome to comments.

I tested this patch with a real app and it works flawlessly.

[sevensolutions]

Hi @Prototik,
thx for submitting this PR.
I'am just thinking if we can find a better way of providing the values.
What immediately came to my mind was simply allowing to specify the same header multiple times.
Eg.

Headers:
  - Name: "Impersonate-Group"
    Value: "developers"
  - Name: "Impersonate-Group"
    Value: "admins"

But the problem is that with this approach it cannot be dynamic.
Your approach allows you to loop over and map every group automatically so i think thats a good solution.

Should we maybe provide a custom helper function so that it's easier to map an array?
Something like:

Headers:
  - Name: "Impersonate-Group"
    Values: '[{{ mapArray(.claim.groups) }}]'

Spoiler: I have no idea yet if this is possible.. Didn't check that yet.

But on the other side, it's an edge case anyway.

[Prototik]

After a bit of tinkering I ended with three additional functions for templates, withPrefix/withSuffix (self-explanatory I guess) and mapToJsonArray, which takes slice of strings and outputs valid json directly, without manual tinkering. So current example is

  - Name: "Impersonate-Group"
    Values: '{{ .claims.groups | withPrefix "prefix:" | mapToJsonArray }}'

Or in imperative style:

  - Name: "Impersonate-Group"
    Values: '{{ mapToJsonArray(withPrefix("prefix:", .claims.groups)) }}'

[sevensolutions]

Many thanks @Prototik, great work 👍
Just one small thing we should check and then I'am happy to merge it.

[sevensolutions]

Thank you @Prototik, looks good now.
Please give me a 👍 when you're happy with it and I'll merge it.