Update loadbalancers service to SDK v2

[nawazkh]

part of #3409

[nawazkh]

/assign

[mboersma]

My approach to these has basically been:

• Change the import line to reference the new SDK v2 package
• Replace network. with armnetwork. (for example) in the loadbalancers.go file
• Fix the resulting compilation errors according to SDK v2 changes
• Repeat the above three steps for the remaining files in the package
• Update the client.go file following the pattern of other SDK v2 PRs
• Run make generate-go lint go-test
• Fix errors and repeat

That should do most of it, but in this case there's some special behavior to replicate. The current client adds the If-Match header when the loadbalancer has an ETag set.

We don't have direct access to the request or its "preparer" in SDK v2, so we have to add a custom Policy class whose behavior can be controller from within the CreateOrUpdateAsync func. The CustomPutPatchHeaderPolicy is one example of a custom policy.

[nojnhuh]

That should do most of it, but in this case there's some special behavior to replicate. The current client adds the If-Match header when the loadbalancer has an ETag set.

I added something to do this in the second commit of #3938