Add support for `restrict_update` to Rulesets convenience rules

[ppkarwasz]

It would be useful to add a restrict_update key to the convenience syntax for Rulesets to govern the presence or absence of the update rule.

This rule is not very useful for branches, but it should probably default to true for tags, making them effectively immutable.

[potiuk]

I am supportive of it - I hppe others who have the "write" access to the repo can chime in .