Fix issue with user defined functions

[rylev]

Creating user defined functions was broken - this fixes that and makes a test for user defined functions using the mock framework.

The fix itself was twofold:

• Using the right format for the date header. I actually think all of the cosmos crate is broken since we don't test dates in local tests since they change on every request. I believe this was an issue introduced by switch from chrono crate to time crate #965.
• The resource type (which is used to sign the request) was not correct for user defined functions. There was one additional place where the resource type was incorrect - which I fixed. We're missing test coverage for stored procedures unfortunately.

Additionally, I made the following changes:

• We currently only retry HTTP errors that are not on the disallow list and all other errors. This makes it such that only IO errors get retried. Generally speaking, any other type of error is something wrong that a retry won't fix, and it should be bubbled up to the user ASAP.
• There were a few general headers that were duplicated across azure_core and azure_data_cosmos. I deleted the ones in cosmos and just used the ones from azure_core.

[cataggar]

Yes, looks like this was missed in #965.