Add HasCharSet method for ComplexPropertyBuilder

[trejjam]

Hi, this aims to fix an invalid snapshot generated when setting the charset on a ComplexProperty property.

Snapshot.cs(443,71): error CS1503: Argument 1: cannot convert from 'Microsoft.EntityFrameworkCore.Metadata.Builders.ComplexTypePropertyBuilder<string>' to 'Microsoft.EntityFrameworkCore.Metadata.Builders.PropertyBuilder'

b.ComplexProperty<Dictionary<string, object>>("Figure", "...Entity.Figure#File", b1 =>
{
    b1.IsRequired();

    b1.Property<string>("Path")
        .IsRequired()
        .HasMaxLength(256)
        .HasColumnType("varchar(256)")
        .HasColumnName("FigurePath");

        MySqlPropertyBuilderExtensions.HasCharSet(b1.Property<string>("Path"), "utf8mb4");
});

[lauxjpn]

Yes, adding HasCharSet() extension methods for complex type properties makes sense.

A current workaround should be to just specify a collation instead of a charset (which is more precise anyway).

[lauxjpn]

In addition to the comments below, I think the ComplexTypePropertyBuilder extension methods should get their own class/file.

[lauxjpn]

Just a little cleanup, otherwise it looks good!

[lauxjpn]

LGTM!

[lauxjpn]

@trejjam Thank you for your contribution!