feat(gateway): optionally compute instrospectionHeaders per request via a function

[trevor-scheer]

Ref

• fix(gateway): Make protected fields and methods private (except loadServiceDefinitions) #539 (comment)
• fix(gateway): Make protected fields and methods private (except loadServiceDefinitions) #539 (comment)

Relates to #563, though the change I'm recommending in this issue is a bit more short-sighted (still useful!)

It's reasonable to need to calculate introspectionHeaders dynamically / per request. Rather than reaching into the gateway's config object to modify them, we can provide an API for this. See linked comments for additional context.

[williamboman]

I'll take a look at this!

[williamboman]

#607