Conventions: Add public model building API

[AndriySvyryd]

The InternalModelBuilder has several problems:

• It is a leaky abstraction, as it exposes ConfigurationSource and requires the users to keep track of the last returned builder that contains the up-to-date configuration.
• Its shape resembles ModelBuilder, but the behavior is different, sometimes in non-obvious ways
• The API is inconsistent in parameters as it can take string ids, Model objects or InternalModelBuilder objects.

[AndriySvyryd]

One approach that could solve these issues is the following:

1. Make InternalModelBuilder the only source-of-truth while the model is being built.
2. Make all users go through one of the interfaces that the InternalModelBuilder would implement:

• `IMutableModel`, would have mostly the same shape as `Model` currently has, but would also have overloads that use names for element identification (e.g. `IMutableKey AddKey(IEnumerable)`). Can be dropped down to from `ModelBuilder`.
• `IConventionModel` would have a similar shape to IMutableModel, while conveying that configuration is not assured (e.g. `IConventionKey TryAddKey(IEnumerable, ConfigurationSource)`, `TrySetRequired(bool, ConfigurationSource)`).
• These interfaces would derive from `IModel` and `IAccessor`
• `ModelBuilder`: conflicting configuration - last one wins, invalid configuration - throws
• `IMutableModel`: conflicting configuration - throws, invalid configuration - throws
• `IConventionModel`: conflicting configuration - last one wins, invalid configuration or conflict with explicit configuration - returns `null`

3. Add 3 more values to `ConfigurationSource`: `WhatIfExplicit`, `WhatIfDataAnnotation`, `WhatIfConvention`. When used they would not perform any changes, but would return the same success result as the equivalent 'real' sources. 4. When the model is built and the `ModelValidator` ran successfully it can be converted to `ReadOnlyModel`, that only implements IModel and is optimized for reading speed and low memory consumption.

[AndriySvyryd]

Marking for retriage.
I think this should be pri0 since it would introduce a big API surface and early release can generate helpful feedback.
Also it will make other changes and fixes in the builders easier as there would be less code duplication.

[rowanmiller]

@AndriySvyryd can you bring a proposal to the design meeting... or a corridor meeting in our corridor-less open space

[AndriySvyryd]

We've discussed this and decided to keep the model and the model building logic separate. To improve perf ConfigurationSource will be stored as part of the model, see #3476

This issue is now only for adding public model building API for conventions.

[rowanmiller]

Custom conventions is tracked by #214, so closing this out.