LogpushFilter support

[yknx4]

Adds support for filter field in LogpushJob as set in here

Description

• Added new structs for Filters
• Added Operator enums for valid operators
• Added helper function to attach filter (since it is a json string and not a json object)
• Added a validation to ensure filters are sound

Has your change been tested?

Added struct test that matches example in Cloudflare docs

Types of changes

What sort of change does your code introduce/modify?

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• This change is using publicly documented (api.cloudflare.com or developers.cloudflare.com) and stable APIs.

[jacobbednarz]

this PR looks great, thank you! my only hesitation at the moment is the job.AddFilter call. within the rest of the library, we put the filtering parameters directly on the struct itself without a need for the additional method. example:

job := cloudflare.LogpushJob{
	Name:            "example.com static assets",
	LogpullOptions:  "fields=RayID,ClientIP,EdgeStartTimestamp&timestamps=rfc3339&CVE-2021-44228=true",
	Dataset:         "http_requests",
	DestinationConf: "s3://<BUCKET_PATH>?region=us-west-2/",
	Filter: cloudflare.LogpushJobFilters{
		Where: cloudflare.LogpushJobFilter{
			And: []cloudflare.LogpushJobFilter{
				{Key: "ClientRequestPath", Operator: cloudflare.Contains, Value: "/static\\"},
				{Key: "ClientRequestHost", Operator: cloudflare.Equal, Value: "example.com"},
			},
		},
	}
}
    

however, taking this approach, we'll need a custom marshal handler which may be annoying. WDYT?

[yknx4]

@jacobbednarz It makes a lot of sense to keep the consistency of the library. Let me do some refactoring to add custom marshalling for that field and an extra test to ensure bot Marshalling and Unmarshalling works as expected

[jacobbednarz]

awesome, thanks! other than that, the testing and approach looks ace.

[yknx4]

@jacobbednarz Refactoring done ~

[jacobbednarz]

thank you for the contribution, you rock! 🥇