DefaultApiConventions are missing Status400BadRequest responses for GET requests

### Describe the bug
The [DefaultApiConventions](https://github.com/aspnet/AspNetCore/blob/master/src/Mvc/Mvc.Core/src/DefaultApiConventions.cs) should define a `Status400BadRequest` response for `Get` and `Find` methods.

### To Reproduce
Steps to reproduce the behavior:
1. Using this version of ASP.NET Core '2.2'
2. Create a Web API project
3. Apply the `[ApiConventionType(typeof(DefaultApiConventions))]` attribute to the controller or assembly.
4. Analyse the structure returned by `ApiExplorer` interfaces (or add [Swashbuckle.AspNetCore](https://www.nuget.org/packages/Swashbuckle.AspNetCore) to quickly see the same data in Swagger-UI).
5. Check the possible responses of `api/values/{id}` methods. Note that `Status400BadRequest` is not defined as a possible response.
6. Call the `api/values/{id}` with a string as `id`. Note the response ("400 BadRequest").

### Expected behavior
Becaues GET can return "400 BadRequest", there should be a `Status400BadRequest` response defined for GET methods, just like there is for all POST, PUT and DELETE.

### Additional context
I know that these conventions were created after the default scaffolding template "Web API Controller with EF Core context", which doesn't return `BadRequest` in `Get`. But neither does `Delete`, which accepts the same parameter as `Get`. And thanks to [automatic HTTP 400 responses](https://docs.microsoft.com/en-us/aspnet/core/web-api/index?view=aspnetcore-2.2#automatic-http-400-responses) both `Get` and `Delete` will return "400 BadRequest" if model binding fails (e.g. if you call the action with a string parameter but your id is an integer).

Output of `dotnet --info`:

Click to expand
.NET Core SDK (gemäß "global.json"):
Version: 2.2.106
Commit: aa79b139a8

Laufzeitumgebung:
OS Name: Windows
OS Version: 10.0.16299
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\2.2.106\

Host (useful for support):
Version: 2.2.4
Commit: f95848e524

.NET Core SDKs installed:
2.1.505 [C:\Program Files\dotnet\sdk]
2.2.106 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.1.9 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.2.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.9 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.2.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.1.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.2.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
https://aka.ms/dotnet-download