Enable retrieval of historical data for 'target'

[merav-aharoni]

Summary

Allow the user to retrieve historical data for the target of the backend.

Details and comments

This is similar to the support that already exists for backend.properties. For target, we defined a new method - target_history, since target is defined as a property.
Note that when we try to pass the datetime parameter to defaults(), it does not work, because there is no support for this parameter on the server side.
See issues #484, #77 .

[merav-aharoni]

@kt474 I implemented usage of datetime in both IBMBackend.target() and IBMBackend.defaults(). However, when I look at the results obtained from a backend (Manila) with a datetime of about a year ago compared to results from now, the results seem identical both for defaults and for target. I think this means something is wrong. However, I have no idea what the problem can be. Can you please have a look?

[coveralls]

Pull Request Test Coverage Report for Build 4377155810

• 2 of 9 (22.22%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 50.077%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit_ibm_provider/ibm_backend.py	2	9	22.22%

Totals
Change from base Build 4377096380:	0%
Covered Lines:	2942
Relevant Lines:	5875

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💛 - Coveralls

[kt474]

LGTM - can you add a release note

[merav-aharoni]

Added the release note (had it ready, but forgot to push).