add http annotation to fragment proto

[parthea]

Updating fragment proto to reproduce error in googleapis/python-debugger-client#151.

See build log here.

From googleapis/python-debugger-client#151,

E       ValueError: Invalid request.
E       Some of the fields of the request message are either not initialized or initialized with an invalid value.
E       Please make sure your request matches at least one accepted HTTP binding.
E       To match a binding the request message must have all the required fields initialized with values matching their patterns as listed below:
E       	URI: "/v2/debugger/debuggees/{debuggee_id}/breakpoints/set"
E       	Required request fields:
E       		field: "debuggee_id", pattern: "*"

[parthea]

I've proposed a fix in googleapis/python-api-core#489