Introduce environment variables for MaxMessageSize and MaxGroupSize settings

[jorgebay]

MaxMessageSize() and MaxGroupSize() config settings are hardcoded.

We should use environment variables to allow users to set it to the desired size, if needed.

See MaxSegmentSize() for example

• https://github.com/polarstreams/polar/blob/v0.6.0/internal/conf/config.go#L37
• https://github.com/polarstreams/polar/blob/v0.6.0/internal/conf/config.go#L311-L313

[sunilkumardash9]

I suppose this is what intended to be done

[jorgebay]

@sunilkumardash9 yeah!

[sunilkumardash9]

Would love to contribute

[jorgebay]

Sure! I'll assign it to you 👍