Investigate: Use a file format for dotnet-trace profiles

Right now we have hard-coded profile names that are accepted by the --profile argument in dotnet-trace. We should look into defining a simple file format where provider/keyword/level/filter info can be written. The --profile argument would then be interpreted as the name of one of these files. This makes the list extensible by end-users.

My expectation if we went down this route is that Microsoft would still provide some pre-defined options (perhaps now written as files instead of constants in the code). Most users would probably stick to that set, but experts or adventurous users might create their own.

Some open questions:
a) What is the file format?
b) Does --profile support/require a full path?
c) Assuming it doesn't require full path, where does the tool look to find the files?
d) Do we need a way to enumerate the options?
e) Should the file contain descriptive information that users can see, either by directly looking at the file or by having the UI show it somewhere?
f) Does adding these files subsume the need to specify providers on the command-line?