feat(metadata): implement GET Units of Measure API

[chr1shung]

dependent on edgexfoundry/go-mod-core-contracts#756

cbc81fb updated uom.toml configuration structure in order to return similar content for json/toml response.

fix #3957

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PR Checklist

Please check if your PR fulfills the following requirements:

• I am not introducing a breaking change (if you are, flag in conventional commit message with BREAKING CHANGE: describing the break)
• I am not introducing a new dependency (add notes below if you are)
• I have added unit tests for the new feature or bug fix (if not, why?)
• I have fully tested (add details below) this the new feature or bug fix (if not, why?)
• I have opened a PR for the related docs change (if not, why?) The API document is also updated in this PR.

Testing Instructions

1. Run core-metadata from this branch

$ curl localhost:59881/api/v2/uom
{
    "apiVersion": "v2",
    "statusCode": 200,
    "uom": {
        "source": "reference to source for all UoM if not specified below",
        "units": {
            "temperature": {
                "source": "www.weather.com",
                "values": [
                    "C",
                    "F",
                    "K"
                ]
            },
            "weights": {
                "source": "www.usa.gov/federal-agencies/weights-and-measures-division",
                "values": [
                    "lbs",
                    "ounces",
                    "kilos",
                    "grams"
                ]
            }
        }
    }
}

$ curl -H "Accept: application/toml" localhost:59881/api/v2/uom
Source = "reference to source for all UoM if not specified below"

[Units]

  [Units.temperature]
    Source = "www.weather.com"
    Values = ["C", "F", "K"]

  [Units.weights]
    Source = "www.usa.gov/federal-agencies/weights-and-measures-division"
    Values = ["lbs", "ounces", "kilos", "grams"]

New Dependency Instructions (If applicable)

[lenny-goodell]

@hahattan , I noticed the I am not introducing a breaking change isn't checked in the PR checklist above. I assume this was just missed and there are not breaking changes :-)

[chr1shung]

@hahattan , I noticed the I am not introducing a breaking change isn't checked in the PR checklist above. I assume this was just missed and there are not breaking changes :-)

not sure whether updating the uom.toml is a breaking change so I left it unchecked

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
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No Coverage information
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[cloudxxx8]

LGTM

[cloudxxx8]

@hahattan , I noticed the I am not introducing a breaking change isn't checked in the PR checklist above. I assume this was just missed and there are not breaking changes :-)

not sure whether updating the uom.toml is a breaking change so I left it unchecked

it's not a breaking change, because this file didn't exist in the previous release

[lenny-goodell]

LGTM