Add sprig template library

[hypnoglow]

Proposed changes

This commit adds sprig template library functions to the templates used in header/cookie mutator.

Go stdlib templates are very limited in terms of features. For example, I have a use case where I have an array of strings in session data, and I want to use mutator to join them into the single header string. Currently it is kind of impossible to do properly. This template almost does this, but there is no possibility on removing trailing comma:

{{- $claims := (index .Extra "example.com/some-claims") -}}
{{- range $claim := $claims }}
{{- $claim -}},
{{- end -}}

With Extra set to map[string]interface{}{"example.com/some-claims":[]string{"Foo","Bar"} it compiles into Foo,Bar,.

With sprig library we could replace the whole template with simple {{- (index .Extra "example.com/some-claims") | join "," -}} and it will produce correct output Foo,Bar.

Sprig can also handle many other advanced use cases.

Checklist

• I have read the contributing guidelines
• I have read the security policy
• I confirm that this pull request does not address a security vulnerability. If this pull request addresses a security
  vulnerability, I confirm that I got green light (please contact [email protected]) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation within the code base (if appropriate)
• I have documented my changes in the developer guide (if appropriate)

[aeneasr]

This is a great idea!