Adding basic GridFS support

[rohitkumbhar]

Adding basic GridFS support.

@dstepanov I'm not sure if it's desired to have it but I have opened a PR just in case it is.

[dstepanov]

Sorry for getting late to it. It looks good, but the configuration should be done to each Mongo Client defined. It might be better to contribute it to Micronaut MongoDB.

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[Stoorx]

+1 to this PR (and the idea of GridFS support in general)

But I had a glance at the changes and I definitely do not like the way the files are up/downloaded from/to the local filesystem. I have checked how it is made in the Spring Framework, and they just use Input/OutputStreams. Maybe you (@rohitkumbhar) should take a look at the Spring Data code for some inspiration.

Also, (in response to @dstepanov's comment) I am not sure whether it is reasonable to move GridFS support to the micronaut-mongodb project, since it is more logically coupled with micronaur-data functionality. But I could mistake or misunderstand your point.