add regressor package for curvefit extension

[sunya-ch]

This PR works on the issue #1244 for curvefit extension.
I introduce a new package called regressor to replace LinearRegressor.
LinearRegressor is moved into this package as well as other new functions from curvefit trainer (check kepler-model-server issue).

The notice point:

• trainer name needed to specify to use other curvefit function (using already-exists TRAINER key defined under modelConfig).
• linear regressor has different weight definition since it can support multiple metrics while curvefit won't.
• Curvefit will use only a single feature value. This is defined in the weight scale.
• For log function, need update curvefit (log func) and generate_spec  kepler-model-server#232 to merged first.

Signed-off-by: Sunyanan Choochotkaew sunyanan.choochotkaew1@ibm.com

[sunya-ch]

Got this error after first golangci failed.

It seems the problem of cache: golangci/golangci-lint-action#244.
May I add skip-pkg-cache: true to PR CI.

Now working on golangci fix.

[rootfs]

@sunya-ch @marceloamaral there are some lint warning (duplicate code). Let's merge the PR right now for 0.7.4 and remove the duplicate code (if still exists after pkg cache CI fix) in a follow up PR in 0.7.5