c# feature(RepeatedField): Capacity property to resize the internal array

[prat0088]

This property is useful for efficiently initializing long RepeatedFields.

I used System.Collections.Generic.List's design as a model. It should feel the most familiar to C# developers.

Fixes: #5501

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[prat0088]

@googlebot I signed it!

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[WilliamWhispell]

I need this feature for my project. @anandolee - what is needed to get this merged?

[jtattermusch]

LGTM once the tests pass.