perf!: change ValidationRuleContext to readonly struct

[skwasjer]

• no longer need null-reference checks
• adds IEquatable<> which can help to make tests simpler
• moves responsibility to set country code away from rule and makes ctx immutable
• reduces heap allocations slightly (but very small)

It is however a breaking change.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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