This adds support for using templated/dynamic urls in routes.
* refactor interpolateString into utils and add interpolation support for app plugin routes.
* cleanup and add error check for url parse failure
* add docs for interpolated route urls
Closes#16835
See,
$ gometalinter --vendor --deadline 10m --disable-all --enable=golint ./...
ip.go:8:6⚠️ func SplitIpPort should be SplitIPPort (golint)
url.go:14:6⚠️ func NewUrlQueryReader should be NewURLQueryReader (golint)
url.go:9:6⚠️ type UrlQueryReader should be URLQueryReader (golint)
url.go:37:6⚠️ func JoinUrlFragments should be JoinURLFragments (golint)
