Import the siteinfo package from m-lab/switch-monitoring

[robertodauria]

This PR imports the siteinfo package from github.com/m-lab/switch-monitoring as a new package in alpha state.

Since I'm going to use it in another project, I figured out it would be a good time to move it here rather than having a second project that depends on switch-monitoring.

The code is copied 1-1 and likely does not need another review, except for the fact I've copied the HTTPProvider interface from switch-monitoring/internal/interfaces.go to siteinfo/client.go here.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 810

• 28 of 28 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 81.877%

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Totals
Change from base Build 801:	0.3%
Covered Lines:	1396
Relevant Lines:	1705

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💛 - Coveralls