Add command-line flags to set model memory limit

[samikama]

This PR adds a command-line flag to expose total model memory limit parameter to user. For ease of use limit is expressed in MB units and converted to bytes internally when passed to model server.

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[samikama]

@googlebot I signed it!

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[samikama]

@ebrevdo since you made the most recent commit, any idea who to ping for this PR or opinions on this? Thanks.

[netfs]

set default to 0

[netfs]

update total_model_memory_limit_bytes only when server_options.total_model_memory_limit_megabytes is not zero (implying someone changed the value via command line). this ensures that default limit continues to be applied as before. your current change assumes that the default is unit64_max.