feat: adds get sign template and get sign templates methods on Client

[dtrodger]

Tell @PJSimon Dave Rodgers says hi :)

[coveralls]

Pull Request Test Coverage Report for Build 5831713999

• 15 of 16 (93.75%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.001%) to 93.526%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
boxsdk/client/client.py	9	10	90.0%

Files with Coverage Reduction	New Missed Lines	%
boxsdk/client/client.py	1	86.23%

Totals
Change from base Build 5725071206:	0.001%
Covered Lines:	3467
Relevant Lines:	3707

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💛 - Coveralls

[congminh1254]

Hi @dtrodger,
I have just added the tests for this sign templates, but it turned out that the response body from API is missing type field, it could make some issue while serialise the object.
So I'm contacting with the team who own this API to see if they can add that field to the request body, then we can merge this PR.
Thanks for your patience.
Regards,
Minh