Custom headers on HTTP requests

[mharvey-jt]

For observability purposes we would like to be able to annotate all HTTP requests with headers that indicate the source of read. This will be a mix of statically-assigned config data, and request-specific, eg the user id, pid, which are available from the fuse request.

eg in config file

CUSTOM_HEADERS=h1=foo1,h2=foo2

and headers in the HTTP request:

X-CVMFS-HEADER-h1: foo1
X-CVMFS-HEADER-h2: foo2
X-CVMFS-HEADER-uid: <uid>
X-CVMFS-HEADER-gid: <gid>
X-CVMFS-HEADER-pid: <pid>