Add support for `$sampleRate` aggregation operator

[christophstrobl]

Available since MongoDB 4.4.2.

• https://docs.mongodb.com/manual/reference/operator/aggregation/sampleRate/

[JamesMcNee]

Hi @christophstrobl I had a crack at this (see PR), hope that is ok? First time contributing to Spring but I think I have followed all of the contributing guidelines!

[christophstrobl]

@JamesMcNee thanks for the PR! Please pardon delayed review/responses during the summer/holiday season. Thanks for your understanding! We'll have a look asap.

[JamesMcNee]

@christophstrobl Absolutely no worries! Enjoy the holidays! 🌴

[JamesMcNee]

Realised that I had actually implemented this incorrectly. Will raise as a new PR when complete, apologies.

[JamesMcNee]

New PR raised here. Looking forward to any feedback when you have time and are back from the summer holidays 😄