Add @ConditionalOnEnumProperty

[snicoll]

We allow case insensitive enum values in the Environment but @ConditionalOnProperty does not really support that.

There is already one usage in the code base, see TestDatabaseAutoConfiguration.

See also #6083

[snicoll]

Turns out we can implement #6083 in a different way. TestDatabaseAutoConfiguration uses properties mapping so the case will be right.