Changes to Account and Person for identity verification

[remi-stripe]

This adds new properties and parameters to reflect the verification state of an Account and a Person:

• company[verification] on Account to support passing a document to verify a business entity.
• verification[additional_document] on Person for cases where a second document might be required for identity verification.
• company[tax_id_registrar on Account which was also missing
• more documentation added while I touched those files

r? @ob-stripe
cc @stripe/api-libraries

This is WIP because the tests are failing on the latest stripe-mock similarly to stripe/stripe-java#842

[xUnit.net 00:00:01.35]     StripeTests.AccountTest.DeserializeWithExpansions [FAIL]
Failed   StripeTests.AccountTest.DeserializeWithExpansions
Error Message:
 StripeTests.StripeTestException : Couldn't reach stripe-mock at `localhost:12111`. Is it running? Please see README for setup instructions.
Stack Trace:
   at StripeTests.StripeMockFixture.GetFixture(String path, String[] expansions) in ./src/StripeTests/StripeMockFixture.cs:line 90
   at StripeTests.BaseStripeTest.GetFixture(String path, String[] expansions) in ./src/StripeTests/BaseStripeTest.cs:line 168
   at StripeTests.AccountTest.DeserializeWithExpansions() in ./src/StripeTests/Entities/Accounts/AccountTest.cs:line 36
[xUnit.net 00:00:06.47]     StripeTests.Issuing.CardTest.Deserialize [FAIL]
Failed   StripeTests.Issuing.CardTest.Deserialize
Error Message:
 Assert.NotNull() Failure
Stack Trace:
   at StripeTests.Issuing.CardTest.Deserialize() in ./src/StripeTests/Entities/Issuing/Cards/CardTest.cs:line 24

[ob-stripe]

Tests were failing because the stripe-mock fixtures don't have the new attributes. Given that fixtures are unstable and subject to change each time we regenerate them I've just removed the expansion tests. Not ideal but I don't think there's a better solution for now.

[ob-stripe]

Released as 28.11.0.