Implement JsonSchemaAs for OneOrMany instead of JsonSchema

[swlynch99]

Currently, there is an impl JsonSchema for WrapSchema<OneOrMany<...>>. This works fine if you only directly need OneOrMany but causes errors if you want to nest it (e.g. Option<OneOrMany<_>>).

This PR changes the impl to a JsonSchemaAs impl, which is the correct approach. I think I just missed these ones during a rebase. In any case, the change should be fully backwards compatible due to the blanket schema impl.

I have added a snapshot test. The actual snapshot doesn't matter - the real test is that the struct below compiles

#[serde_as]
#[derive(Serialize, Deserialize, JsonSchema)]
struct Test {
    #[serde_as(as = "Option<OneOrMany<_>>")]
    optional_vec: Option<Vec<String>>,
}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.25%. Comparing base (1e4481c) to head (dee706a).
Report is 21 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #760      +/-   ##
==========================================
+ Coverage   66.55%   67.25%   +0.69%     
==========================================
  Files          38       38              
  Lines        2473     2452      -21     
==========================================
+ Hits         1646     1649       +3     
+ Misses        827      803      -24     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[jonasbb]

Thanks, that looks good :)