Read #[validate(...)] attributes

[GREsau]

Fixes #12

This PR makes #[derive(JsonSchema)] consume #[validate(...)] attributes that would typically be set when using the validator crate. In particular, it accepts:

• #[validate(email)]/#[validate(phone)]/#[validate(url)] - sets the schema's format to "email"/"phone"/"uri" respectively
• #[validate(length(min = 1, max = 100)]/#[validate(length(equal = 100)] - for string schemas, sets the minLength/maxLength properties. For Array schemas, sets the minItems/maxItems properties.
• #[validate(range(min = 1, max = 100)] - for numbers schemas, sets the minimum/maximum properties
• #[validate(regex = "path::to::regex"]) - for string schemas, sets the pattern property
• #[validate(contains = "str"]) - for string schemas, sets the regex-escaped value to the pattern property (unless regex is also used). For object schemas, adds the value to the "required" property
• #[validate(required)]/#[validate(required_nested)] creates a schema as though the field was not an Option

These attributes are only consumed on structs with named fields - not on tuple structs or newtype structs.

This PR does NOT handle these attributes available in the validator crate:

• must_match
• credit_card
• custom
• nested

[liamcurry]

This is great, thank you! Are there plans to support tuple/newtype structs as well?

Also, would it be possible to support inline patterns with the regex attribute? e.g. #[validate(regex = "^\w+$")]

[GREsau]

Are there plans to support tuple/newtype structs as well?

Yep!

Also, would it be possible to support inline patterns with the regex attribute? e.g. #[validate(regex = "^\w+$")]

Yeah I'd like to do that - the validator crate that these attributes are based off of doesn't support inline patterns, although I did open a feature request for it recently: Keats/validator#149
The syntax would need to be a bit different to the "normal" regex attribute so that it's clear whether it's referring to a variable or an inline pattern. In the issue I suggested #[validate(regex(pattern = r"^[Hh]ello\b"))]. Alternatively, maybe an inline regex could be indicated by surrounding it with / characters e.g. #[validate(regex = r"/^[Hh]ello\b/")]

[liamcurry]

Is it important for the feature be added to the validator crate first, or can schemars experiment with custom #[validate] attributes? I would love to see the regex(pattern = ..) syntax you suggested in schemars.