Allow easy customization of EmbeddedMongo DownloadConfig

[mgmeiner]

This PR introduces 2 new configuration properties to allow easy customization of the DownloadPath of a mongodb.

• spring.mongodb.embedded.download.path = custom download url
• spring.mongodb.embedded.download.user-agent = custom User-Agent HTTP-Header for the download request

We need this due to some special security rules in our company which prevents us from downloading the binaries directly from mongodb. With this PR it is possible to easy override the default download-path and point it to our own server.
Also it is now possible to easy create a completely custom DownloadConfig by simply providing a bean which implements IDownloadConfig

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[wilkinsona]

@mgmeiner Thanks very much for opening your first contribution to Spring Boot. Given that, as far as we know, you requirements are fairly unusual, I wonder if the new properties are justified. To reduce the surface area of the changes I think it might be better to only offer a bean-based solution. That could either be the conditional IDownloadConfig bean that you have proposed or we could offer a DownloadConfigBuilderCustomizer callback interface. Beans implementing that interface would then be called by EmbeddedMongoAutoConfiguration prior to creating the download config. Unlike providing an IDownloadConfig bean, this would avoid the need for user code to deal with configuring the progress listener. What do you think?

[mgmeiner]

@wilkinsona I like your idea on this and you're right: introducing new properties for the download config which is indeed a very specific case - is not justified. I updated the PR and implemented it how you suggested. I named the Customizer EmbeddedMongoDownloadConfigBuilderCustomizer to match the naming of the other classes in the package.

[mgmeiner]

not sure why the tests are failing.. Locally all tests have passed without error

[wilkinsona]

That's a known issue that may be caused by an underlying bug in the JDK. I'd like Embedded Mongo to retry in that case, but the suggestion didn't get any traction.

[snicoll]

@mgmeiner thank you for making your first contribution to Spring Boot. This is now merged in master with a polish commit.