Rename BNS related endpoints

[zone117x]

The API implements a set of BNS endpoints. The URL paths are currently a holdover from Stacks 1.0 and have caused some confusion. They should be renamed to exist under a prefix like /bns.

Here's the list of endpoints:

[
  "/v2/prices/namespaces/:namespace",
  "/v2/prices/names/:name",
  "/v1/namespaces",
  "/v1/namespaces/:tld/names",
  "/v1/names/:name/zonefile/:zoneFileHash",
  "/v1/names/:name/subdomains",
  "/v1/names/:name/zonefile",
  "/v1/names",
  "/v1/names/:name",
  "/v1/addresses/:blockchain/:address"
]

This will be a breaking change unless we keep redirects in place.