Define onlyIfUnchanged parameter for CreateOrUpdate and Delete operations

[heaths]

Instead of defining an accessConditions parameter that takes an IfMatch or IfNoneMatch property (mutually exclusive), we have decided to define a boolean onlyIfUnchanged parameter that defaults to false for CreateOrUpdate and Delete operations on SearchServiceClient for models that define an ETag.

The idea is to make these methods simple to consume without understanding a lot of how the HTTP headers must be set. This same design was inspired from AppConfiguration.

For CreateOrUpdate, this is straight forward: we already take entire models that have an ETag, so you just get that and, if onlyIfUnchanged is true, pass the If-Match header.

For Delete, which currently takes only a string name + accessConditions, we'll make the following changes:

• Keep the string name overload, but remove accessConditions. This will unconditionally delete resources.
• Define an overload that takes the full model (like CreateOrUpdate) and defines the boolean onlyIfUnchanged.