Revisit model-snapshot for perf

[ajcvickers]

At the beginning of EF Core we decided that we would use the fluent API in model snapshots rather than designing a new serialization format. (In other words, the fluent API is the way to serialize the model.) However, this can lead to a lot of code compiled for big models and when the number of migrations build up. See, for example, #18618.

We should consider:

• A different snapshot serialization, potentially based on the compiled model (when it exists, see Reduce EF Core application startup time via compiled models #1906)
• Mitigate by supporting squash for migrations: Squash migrations #2174
• Some more extensive overhaul of the migrations experience

[AndriySvyryd]

Note that this will likely mean that snapshots will need to be updated for major version upgrades and possibly minor ones. So we should consider #18557 at the same time

[bricelam]

Another idea I has was to only put the store model in the *.Designer.cs files, but this doesn't work since we need the full model when removing a migration.

[ajcvickers]

We believe this is best handled by squashing migrations: #2174

[ajcvickers]

Re-open to consider using the relational model.