[Backup] Strongly typed policy create/update commands

Looking at the structure of the backup policy object, it seems like a strongly typed create/update command could be created for this scenario.

There are no collections of complex objects, which is typically the reason that JSON objects are used. As far as I can tell, there are only single (albeit polymorphic) objects and some simple collections that could easily be handled with `nargs='+'`.

A true create/update experience would be preferable to a JSON object set experience as policies could be configured in the script rather than JSON files needing to be saved as artifacts. It would also allow for simple updating of existing policies without have to download, modify, save and re-set it.

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### Environment summary

**Install Method:** How did you install the CLI? (e.g. pip, interactive script, apt-get, Docker, MSI, nightly)
Answer here: WGET

**CLI Version:** What version of the CLI and modules are installed? (Use `az --version`)
Answer here: azure-cli (2.0.17+dev)

acr (2.0.11+dev)
acs (2.0.15+dev)
appservice (0.1.16+dev)
backup (1.0.0+dev)
batch (3.1.3+dev)
billing (0.1.4+dev)
cdn (0.0.8+dev)
cloud (2.0.7+dev)
cognitiveservices (0.1.7+dev)
command-modules-nspkg (2.0.1+dev)
component (2.0.7+dev)
configure (2.0.10+dev)
consumption (0.1.4+dev)
container (0.1.10+dev)
core (2.0.16+dev)
cosmosdb (0.1.12+dev)
dla (0.0.11+dev)
dls (0.0.13+dev)
eventgrid (0.1.3+dev)
extension (0.0.2+dev)
feedback (2.0.6+dev)
find (0.2.6+dev)
interactive (0.3.9+dev)
iot (0.1.11+dev)
keyvault (2.0.10+dev)
lab (0.0.10+dev)
monitor (0.0.9+dev)
network (2.0.14+dev)
nspkg (3.0.1+dev)
profile (2.0.12+dev)
rdbms (0.0.6+dev)
redis (0.2.8+dev)
resource (2.0.14+dev)
role (2.0.11+dev)
servicefabric (0.0.3+dev)
sql (2.0.11+dev)
storage (2.0.15+dev)
vm (2.0.14+dev)

**OS Version:** What OS and version are you using?
Answer here: WSL

**Shell Type:** What shell are you using? (e.g. bash, cmd.exe, Bash on Windows)
Answer here: Bash on Windows