feature: cargo-msrv rules (msrv by offset)

[foresterre]

cargo msrv rules add <name from $supportedrules> <values where values in $rule where $rule.id matches name>
... Could not add rule: 'x',
... Caused by: Duplicate, would override existing rule
... Suggestion: "cargo msrv rules remove x", then rerun "cargo msrv rules add x value"

cargo msrv rules remove

and in config, e..g Cargo.toml -> package.metadata.msrv-config

# Define the MSRV as (last release) - 2
[[package.metadata.msrv.rules]]
rule = "count-offset"
value = 2 

# Define the MSRV as 
[[package.metadata.msrv.rules]]
rule = "date-offset"
value = "18 months ago"

Consider:

• Maybe this "rules" section is too generic, and we should just add "msrv by offset" which can be "minus K releases" or "M months ago" (or other units).

• Do we resolve the rules? What do we do with incompatibilities? Can only one of the rules per category exist (assuming there are categories of course :))

• What rules should we want beyond offsets

• Rules are effectively filters