feat: add method to initialize SDK with an httpClient

[jpill]

• Adds the ability to initialize SDK with an HttpClient, and to make modifications to subsequent requests
• Adds cancellation token to client
• Adds HttpResponseMessage to ShipEngineException
• Adds missing markdown
• Redefines several properties as nullable reference types
• Adds and implements IShipEngine interface with virtual mockable ShipEngine

[coveralls]

Pull Request Test Coverage Report for Build 9652824702

Details

• 48 of 142 (33.8%) changed or added relevant lines in 8 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-9.7%) to 75.691%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ShipEngine/Models/Dto/GetRatesFromShipmentDetails/Result.cs	19	23	82.61%
ShipEngine/ShipEngineClient.cs	17	32	53.13%
ShipEngine/ShipEngine.cs	2	77	2.6%
<!--	Total:	48	142	33.8%	-->

Files with Coverage Reduction	New Missed Lines	%
ShipEngine/ShipEngine.cs	1	56.73%
<!--	Total:	1		-->

Totals
Change from base Build 9486504073:	-9.7%
Covered Lines:	644
Relevant Lines:	825

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9664624594

Details

• 48 of 142 (33.8%) changed or added relevant lines in 8 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-9.7%) to 75.691%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ShipEngine/Models/Dto/GetRatesFromShipmentDetails/Result.cs	19	23	82.61%
ShipEngine/ShipEngineClient.cs	17	32	53.13%
ShipEngine/ShipEngine.cs	2	77	2.6%
<!--	Total:	48	142	33.8%	-->

Files with Coverage Reduction	New Missed Lines	%
ShipEngine/ShipEngine.cs	1	56.73%
<!--	Total:	1		-->

Totals
Change from base Build 9486504073:	-9.7%
Covered Lines:	644
Relevant Lines:	825

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9664647461

Details

• 48 of 142 (33.8%) changed or added relevant lines in 8 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-9.7%) to 75.691%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ShipEngine/Models/Dto/GetRatesFromShipmentDetails/Result.cs	19	23	82.61%
ShipEngine/ShipEngineClient.cs	17	32	53.13%
ShipEngine/ShipEngine.cs	2	77	2.6%
<!--	Total:	48	142	33.8%	-->

Files with Coverage Reduction	New Missed Lines	%
ShipEngine/ShipEngine.cs	1	56.73%
<!--	Total:	1		-->

Totals
Change from base Build 9486504073:	-9.7%
Covered Lines:	644
Relevant Lines:	825

---

💛 - Coveralls

[jpill]

closing this PR in favor of https://github.com/ShipEngine/shipengine-dotnet/pull/93