Avoid Option<DeviceType> in all structs

[mhammond]

Fixes #4860

It's unfortunate that anyone who deserializes needs to remember to add the custom default path, but we can't add #[serde(default=...)] to the enum itself because that's only supported for structs. A default deserialize implementation sounds a little like overkill.

Welcome all feedback, so I'm just marking this as a draft now.

[codecov-commenter]

Codecov Report

Base: 46.91% // Head: 47.04% // Increases project coverage by +0.12% :tada:

Coverage data is based on head (f73d3cd) compared to base (b6bbe8c). Patch coverage: 89.70% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4861      +/-   ##
==========================================
+ Coverage   46.91%   47.04%   +0.12%     
==========================================
  Files         180      181       +1     
  Lines       14474    14468       -6     
==========================================
+ Hits         6791     6806      +15     
+ Misses       7683     7662      -21     

Impacted Files	Coverage Δ
components/fxa-client/src/internal/http_client.rs	0.00% <0.00%> (ø)
.../fxa-client/src/internal/oauth/attached_clients.rs	0.00% <0.00%> (ø)
components/fxa-client/src/lib.rs	0.00% <ø> (ø)
components/sync15/src/clients_engine/engine.rs	0.00% <0.00%> (ø)
components/sync15/src/clients_engine/record.rs	0.00% <0.00%> (ø)
components/sync15/src/lib.rs	100.00% <ø> (ø)
components/tabs/src/sync/bridge.rs	0.00% <ø> (ø)
components/tabs/src/sync/engine.rs	0.00% <0.00%> (ø)
components/sync15/src/client_types.rs	98.14% <100.00%> (-0.79%)	:arrow_down:
components/sync15/src/device_type.rs	100.00% <100.00%> (ø)
... and 3 more

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[mhammond]

Having unknown be, say, Unknown(Option<String>) seems ideal, but I'm really not sure how that flies with mobile.

[mhammond]

I update this in the following way:

• Moved DeviceType to a new device_type.rs and added many comments about the semantics for serializing etc.
• Deserialize "phone" as DeviceType::Mobile given https://searchfox.org/mozilla-central/rev/a156a65ced2dae5913ae35a68e9445b8ee7ca457/services/sync/modules/engines/clients.js#292 - I doubt we hit that in practice, but I can't see anything wrong with doing this.

I also tested iOS and Android (although I failed to get Fenix working for reasons unrelated to this. So it seems fine to land.