Add examples for new API

[BrynCooke]

Create an examples directory, include some basic stuff like a plugin and header manipulation.
Show code and YAML examples.

[o0Ignition0o]

• Refactor current plugin examples so they call router::main() instead of building one
• Create an example that does the current examples "main.rs" job.
• add comments explaining that the "build your own router" / "compose your tower::Service" are not required if you only want to write a plugin

[o0Ignition0o]

Extension: if a subgraph returns something specific, change the http status code

Surrogate cache key from fastly ? -> combine em all and join em with commas

Inbound cookie changed from a cookie key / value pair to a header

Cookie : name "Jeremy" => value "foobarbaz" => make it a header "x-jeremy": "foobarbaz"

Go through our homebuilt plugins and document them better

Redacting email address / api keys from response errors```

[abernix]

Closing as these have been broken down into individual issues.