ngclient: max_root_rotations value is very small

[joshuagl]

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Description of issue or feature request:

ngclient (like legacy client?) has a max_root_rotations value of 32, which feels low.

The specification suggests a value of 1024 (2^10)

Can we define a way of calculating a sane default for, i.e., PyPI users and update the default accordingly?

[mnm678]

Per PEP 458, root metadata should expire every year, so we'll want this value to be at least the number of years we anticipate clients will go between updating their local root metadata, plus some extra rotations in case of a compromise. I actually think that 32 should be plenty.