Add support for ffschemaext

[chrisbygrave]

This PR adds support for a gostruct tag ffschemaext that allows a comma-separated list of key/value pairs to be added as extensions to the schema generated into the OpenAPI document. Keys must be of the format x-<name>.

For example, the following structure:

type TestExtensions struct {
	MyString string `ffstruct:"mystruct" json:"mystring" ffschemaext:"x-key=value"`
}

Would generate a schema:

mystring:
  description: mystruct.mystring
  type: string
  x-key: value

Query string syntax (and escaping rules) can be used to add multiple extension to a single field:

type TestExtensions struct {
	MyString string `ffstruct:"mystruct" json:"mystring" ffschemaext:"x-key=value&x-key2=value2"`
}

[EnriqueL8]

Looks good - a question and needs gofmt formatting

[EnriqueL8]

Looks good to me @chrisbygrave - thanks!

[peterbroadhurst]

Couple of thoughts here @chrisbygrave

[chrisbygrave]

Thanks for the steer @peterbroadhurst - I've updated the PR to support query syntax parsing, and added a couple of tests to catch encoding issues.

I've also switched out ffextensions for ffschemaext - it does indeed feel clearer.

[chrisbygrave]

Worth noting that if the same property is specified multiple times, the last value will win - I think that's cleaner behaviour than introducing some arbitrary join character in an attempt to consolidate multiple values down to one.

[peterbroadhurst]

Thanks @chrisbygrave for going round the loop on the spelling and escaping semantics