MetadataClient for Open Source Grafeas

[lynnsh]

This PR will:

• create rest calls to grafeas open source v1beta1 version used by MetadataClient
• create objects to receive and send data used by rest calls (links to similar client-go objects are included in comments)
• add gomock for grafeas open source client tests
• add grafeas open source option to cmd/config MetadataClient setup

Steps to test with a real copy of Grafeas:

• start local grafeas server
• add testing data to the server (either with curl and json or by using GrafeasApiService and createAllOccurrences in client_test)
• test using curl requests or update client_test to use GrafeasApiService configured to call your local grafeas server

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[lynnsh]

@googlebot I signed it!

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[dani-santos-code]

@googlebot I signed it!

[google-cla]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

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[dani-santos-code]

@googlebot I consent

[catherinejones]

I reviewed this in Shopify's codebase and I think everything is good to go!