perf script flamegraph: Avoid d3-flame-graph package dependency

Currently flame graph generation requires a d3-flame-graph template to
be installed. Unfortunately this is hard to come by for things like
Debian [1].

If the template isn't installed then ask if it should be downloaded from
jsdelivr CDN. The downloaded HTML file is validated against an md5sum.
If the download fails, generate a minimal flame graph with the
javascript coming from links to jsdelivr CDN.

v3. Adds a warning message and quits before download in live mode.
v2. Change the warning to a prompt about downloading and add the
    --allow-download command line flag. Add an md5sum check for the
    downloaded HTML.

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=996839



Reviewed-by: Andreas Gerstmayr <agerstmayr@redhat.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: 996839@bugs.debian.org
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Brendan Gregg <brendan@intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Martin Spier <spiermar@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230118072409.147786-1-irogers@google.com # v3 discussion
Link: https://lore.kernel.org/r/20230112220024.32709-1-irogers@google.com # v2 discussion
Link: https://lore.kernel.org/r/CAP-5=fXi_9zdhTAoYApiFQoLURAvpEatFzU3uL23o3zs=z25ZQ@mail.gmail.com

 # v1 discussion
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>