Promote x-ms-text extension to xml's text property

[daviwil]

This change addresses #330 which requests that the x-ms-text extension on a schema property's xml object should be promoted to the CodeModel property XmlSerializationFormat.text. This depends on a CodeModel improvement in Azure/perks#125. I've added a unit test to exercise both the positive and negative cases (setting both text and attribute to true throws an error).

[azure-pipelines]

You may test this build by running autorest --reset and then either:

---

add --use: to the command line:

autorest --use:http://tinyurl.com/yyl5xo6n ...

---

or use the following in your autorest configuration:

use-extension:
  "@autorest/modelerfour": "http://tinyurl.com/yyl5xo6n" 

---

If this build is good for you, give this comment a thumbs up. (👍)

And you should run autorest --reset again once you're finished testing to remove it.

[azure-pipelines]

You may test this build by running autorest --reset and then either:

---

add --use: to the command line:

autorest --use:http://tinyurl.com/y422dlu8 ...

---

or use the following in your autorest configuration:

use-extension:
  "@autorest/modelerfour": "http://tinyurl.com/y422dlu8" 

---

If this build is good for you, give this comment a thumbs up. (👍)

And you should run autorest --reset again once you're finished testing to remove it.

[azure-pipelines]

You may test this build by running autorest --reset and then either:

---

add --use: to the command line:

autorest --use:http://tinyurl.com/y6bl3x25 ...

---

or use the following in your autorest configuration:

use-extension:
  "@autorest/modelerfour": "http://tinyurl.com/y6bl3x25" 

---

If this build is good for you, give this comment a thumbs up. (👍)

And you should run autorest --reset again once you're finished testing to remove it.

[daviwil]

This PR should now be testable, @pakrym could you give it a try with the latest test build: http://tinyurl.com/y6bl3x25

[iscai-msft]

looks great @daviwil, you've done it again! Was able to still generate x-ms-text correctly by just switching one line and this PR doesn't affect any of the other autorest tests

[daviwil]

Awesome, thanks a lot for verifying!